Set flake8 max-doc-length to 79 to enforce smaller line length limit
on docstrings and comments (per coding-style docs).
Updated docs to clarify both requirements are enforced by flake8 and
to remove some leftover language from the pre-black era.
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>
