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/django_stubs_ext/__init__.py

@@ -0,0 +1,3 @@
+from .monkeypatch import monkeypatch
+
+__all__ = ["monkeypatch"]

django_stubs_ext/django_stubs_ext/monkeypatch.py

@@ -0,0 +1,47 @@
+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"]