create monkeypatching function for adding get_item dunder (#526)

* run black

* create monkeypatching function for adding get_item dunder

* whoops i forgot the test

* change the name in INSTALLED_APPS to make test pass

* turn the whole thing into a proper package

* move django_stubs_ext to requirements.txt

* also install requirements.txt

* attempt to fix pre-commit

* numerous small code review fixes

* fix dependency issues

* small dependency fixes

* configure proper license file location

* add the rest of the monkeypatching

* use strict mypy

* update contributing with a note monkeypatching generics

* copy release script from parent package

django_stubs_ext/README.md

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+# Extensions and monkey-patching for django-stubs
+
+[![Build Status](https://travis-ci.com/typeddjango/django-stubs.svg?branch=master)](https://travis-ci.com/typeddjango/django-stubs)
+[![Checked with mypy](http://www.mypy-lang.org/static/mypy_badge.svg)](http://mypy-lang.org/)
+[![Gitter](https://badges.gitter.im/mypy-django/Lobby.svg)](https://gitter.im/mypy-django/Lobby)
+
+
+This package contains extensions and monkey-patching functions for the [django-stubs](https://github.com/typeddjango/django-stubs) package. Certain features of django-stubs (i.e. generic django classes that don't define the `__class_getitem__` method) require runtime monkey-patching, which can't be done with type stubs. These extensions were split into a separate package so library consumers don't need `mypy` as a runtime dependency ([#526](https://github.com/typeddjango/django-stubs/pull/526#pullrequestreview-525798031)).
+
+## Installation
+
+```bash
+pip install django-stubs-ext
+```
+
+## Usage
+
+In your Django application, use the following code:
+
+```py
+import django_stubs_ext
+
+django_stubs_ext.monkeypath()
+```
+
+This only needs to be called once, so the call to `monkeypatch` should be placed in your top-level urlconf.
+
+## Version compatibility
+
+Since django-stubs supports multiple Django versions, this package takes care to only monkey-patch the features needed by your django version, and decides which features to patch at runtime. This is completely safe, as (currently) we only add a `__class_getitem__` method that does nothing:
+
+```py
+@classmethod
+def __class_getitem__(cls, *args, **kwargs):
+    return cls
+```
+
+## To get help
+
+For help with django-stubs, please view the main repository at <https://github.com/typeddjango/django-stubs>
+
+We have a Gitter chat here: <https://gitter.im/mypy-django/Lobby>
+If you think you have a more generic typing issue, please refer to <https://github.com/python/mypy> and their Gitter.
+
+## Contributing
+
+The django-stubs-ext package is part of the [django-stubs](https://github.com/typeddjango/django-stubs) monorepo. If you would like to contribute, please view the django-stubs [contribution guide](https://github.com/typeddjango/django-stubs/blob/master/CONTRIBUTING.md).
+
+You can always also reach out in gitter to discuss your contributions!

django_stubs_ext/django_stubs_ext/__init__.py

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+from .monkeypatch import monkeypatch
+
+__all__ = ["monkeypatch"]

django_stubs_ext/django_stubs_ext/monkeypatch.py

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+from typing import Any, Generic, List, Optional, Type, TypeVar
+
+import django
+from django.contrib.admin import ModelAdmin
+from django.contrib.admin.options import BaseModelAdmin
+from django.views.generic.edit import FormMixin
+
+_T = TypeVar("_T")
+
+
+class MPGeneric(Generic[_T]):
+    """Create a data class to hold metadata about the gneric classes needing monkeypatching.
+
+    The `version` param is optional, and a value of `None` means that the monkeypatch is
+    version-independent.
+
+    This is slightly overkill for our purposes, but useful for future-proofing against any
+    possible issues we may run into with this method.
+    """
+
+    version: Optional[int]
+    cls: Type[_T]
+
+    def __init__(self, cls: Type[_T], version: Optional[int] = None):
+        """Set the data fields, basic constructor."""
+        self.version = version
+        self.cls = cls
+
+
+# certain django classes need to be generic, but lack the __class_getitem__ dunder needed to
+# annotate them: https://github.com/typeddjango/django-stubs/issues/507
+# this list stores them so `monkeypatch` can fix them when called
+_need_generic: List[MPGeneric[Any]] = [
+    MPGeneric(ModelAdmin),
+    MPGeneric(FormMixin),
+    MPGeneric(BaseModelAdmin),
+]
+
+
+# currently just adds the __class_getitem__ dunder. if more monkeypatching is needed, add it here
+def monkeypatch() -> None:
+    """Monkey patch django as necessary to work properly with mypy."""
+    for el in filter(lambda x: django.VERSION[0] == x.version or x.version is None, _need_generic):
+        el.cls.__class_getitem__ = classmethod(lambda cls, *args, **kwargs: cls)
+
+
+__all__ = ["monkeypatch"]

django_stubs_ext/mypy.ini

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+[mypy]
+strict_optional = True
+ignore_missing_imports = True
+check_untyped_defs = True
+warn_no_return = False
+show_traceback = True
+allow_redefinition = True
+incremental = True
+
+plugins =
+    mypy_django_plugin.main
+
+[mypy.plugins.django-stubs]
+django_settings_module = scripts.django_tests_settings

django_stubs_ext/pyproject.toml

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+[tool.black]
+line-length = 120
+include = '\.pyi?$'
+
+[tool.isort]
+line_length = 120
+multi_line_output = 3
+include_trailing_comma = true

django_stubs_ext/pytest.ini

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+[pytest]
+testpaths =
+    ./tests
+addopts =
+    --tb=native
+    -s
+    -v
+    --cache-clear

django_stubs_ext/release.sh

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+#!/usr/bin/env bash
+set -ex
+
+if [[ -z $(git status -s) ]]
+then
+  if [[ "$VIRTUAL_ENV" != ""  ]]
+  then
+    pip install --upgrade setuptools wheel twine
+    python setup.py sdist bdist_wheel
+    twine upload dist/*
+    rm -rf dist/ build/
+  else
+    echo "this script must be executed inside an active virtual env, aborting"
+  fi
+else
+  echo "git working tree is not clean, aborting"
+fi

django_stubs_ext/setup.cfg

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+[flake8]
+exclude = .*/
+select = F401, Y
+max_line_length = 120
+
+[metadata]
+license_file = ../LICENSE.txt

django_stubs_ext/setup.py

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+from distutils.core import setup
+
+from setuptools import find_packages
+
+with open("README.md") as f:
+    readme = f.read()
+
+dependencies = [
+    "django",
+]
+
+setup(
+    name="django-stubs-ext",
+    version="0.1.0",
+    description="Monkey-patching and extensions for django-stubs",
+    long_description=readme,
+    long_description_content_type="text/markdown",
+    license="MIT",
+    url="https://github.com/typeddjango/django-stubs",
+    author="Simula Proxy",
+    author_email="3nki.nam.shub@gmail.com",
+    py_modules=[],
+    python_requires=">=3.6",
+    install_requires=dependencies,
+    packages=["django_stubs_ext", *find_packages(exclude=["scripts"])],
+    classifiers=[
+        "License :: OSI Approved :: MIT License",
+        "Operating System :: OS Independent",
+        "Programming Language :: Python :: 3.6",
+        "Programming Language :: Python :: 3.7",
+        "Programming Language :: Python :: 3.8",
+        "Programming Language :: Python :: 3.9",
+        "Typing :: Typed",
+        "Framework :: Django",
+        "Framework :: Django :: 2.2",
+        "Framework :: Django :: 3.0",
+        "Framework :: Django :: 3.1",
+    ],
+    project_urls={
+        "Release notes": "https://github.com/typeddjango/django-stubs/releases",
+    },
+)

django_stubs_ext/tests/test_monkeypatching.py

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+import django_stubs_ext
+from django_stubs_ext.monkeypatch import _need_generic
+
+django_stubs_ext.monkeypatch()
+
+
+def test_patched_generics() -> None:
+    """Test that the generics actually get patched."""
+    for el in _need_generic:
+        # This only throws an exception if the monkeypatch failed
+        assert el.cls[type] == el.cls  # `type` is arbitrary