[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.

MANIFEST.in

@@ -13,5 +13,4 @@ graft extras
 graft js_tests
 graft scripts
 graft tests
-global-exclude __pycache__
 global-exclude *.py[co]