Without `python_requires`, pip will let you install django-waffle 3.0 on python 3.6 and lower, which are now unsupported. And django-waffle should depend on django, because based on the name of the project, it does, so adding `install_requires` to reflect that.

These changes reduce the amount of linting issues that are silenced by the CI configuration.

* Remove `waffle/south_migrations/` folder from `flake8` exclude list,
  as it no longer exists.
* Migrate unused `waffle/.flake` file content to `setup.cfg`.
* Fix simple linting issues that do not generate side effects.

Django 3.2 introduced the `@admin.action` decorator [1], which simplifies adding permissions and description to admin actions.

As the project requires Django 3.2 at least, since version `3.0.0`, this solution does not require an alternative approach for older Django versions.

[1] https://docs.djangoproject.com/en/3.2/ref/contrib/admin/actions/#the-action-decorator

This is the initial change to start adding typing hints to the project.
It includes:

* Adding `mypy` checks to GitHub actions, and Tox.
* Adding `mypy` command to `run.sh` script.
* Initial `mypy` configuration options in `setup.cfg`.
* Small fixes for existing typing issues.

Introducing partial type hints to the project, in preparation for
publishing typing stubs for any applications using `django-waffle`.

According to [PEP 561](https://peps.python.org/pep-0561/), these should
be the only required changes for this project to start distribution the
type hints, for dependant applications to benefit from.

Adding more missing type hints to the project, mainly for:
* Decorators,
* Models and managers,
* Testutils `override_*` context managers.

This reached EOL on 2023-06-27.