[5.1.x] Migrated setuptools configuration to pyproject.toml.

This branch migrates setuptools configuration from setup.py/setup.cfg to
pyproject.toml. In order to ensure that the generated binary files have
consistent casing (both the tarball and the wheel), setuptools version
is limited to ">=61.0.0,<69.3.0".

Configuration for flake8 was moved to a dedicated .flake8 file since
it cannot be configured via pyproject.toml.

Also, __pycache__ exclusion was removed from MANIFEST and the
extras/Makefile was replaced with a simpler build command.

Co-authored-by: Nick Pope <nick@nickpope.me.uk>

Backport of 4686541691 from main.

docs/topics/auth/passwords.txt

@@ -97,7 +97,7 @@ To use Argon2id as your default storage algorithm, do the following:
 #. Install the :pypi:`argon2-cffi` package. This can be done by running
    ``python -m pip install django[argon2]``, which is equivalent to
    ``python -m pip install argon2-cffi`` (along with any version requirement
-   from Django's ``setup.cfg``).
+   from Django's ``pyproject.toml``).
 
 #. Modify :setting:`PASSWORD_HASHERS` to list ``Argon2PasswordHasher`` first.
    That is, in your settings file, you'd put::
@@ -128,7 +128,7 @@ To use Bcrypt as your default storage algorithm, do the following:
 #. Install the :pypi:`bcrypt` package. This can be done by running
    ``python -m pip install django[bcrypt]``, which is equivalent to
    ``python -m pip install bcrypt`` (along with any version requirement from
-   Django's ``setup.cfg``).
+   Django's ``pyproject.toml``).
 
 #. Modify :setting:`PASSWORD_HASHERS` to list ``BCryptSHA256PasswordHasher``
    first. That is, in your settings file, you'd put::