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

CONTRIBUTING.md

@@ -104,3 +104,9 @@ The workflow for contributions is fairly simple:
 3. make whatever changes you want to contribute.
 4. ensure your contribution does not introduce linting issues or breaks the tests by linting and testing the code.
 5. make a pull request with an adequate description.
+
+## A Note About Generics
+
+As Django uses a lot of the more dynamic features of Python (i.e. metaobjects), statically typing it requires heavy use of generics. Unfortunately, the syntax for generics is also valid python syntax. For instance, the statement `class SomeClass(SuperType[int])` implicitly translates to `class SomeClass(SuperType.__class_getitem__(int))`. If `SuperType` doesn't define the `__class_getitem__` method, this causes a runtime error, even if the code typechecks.
+
+When adding a new generic class, or changing an existing class to use generics, run a quick test to see if it causes a runtime error. If it does, please add the new generic class to the `_need_generic` list in the [django_stubs_ext monkeypatch function](https://github.com/typeddjango/django-stubs/tree/master/django_stubs_ext/django_stubs_ext/monkeypatch.py)