Cython now requires a minimum version. Introduces setup.cfg. Cleans up the CI workflow

[misl6]

• As suggested by @tshirtman in PyPy: Fixes a segfault + add tests #627 we now require a minimum Cython version.
• Build-time dependencies are now completely managed via pyproject.toml
• extras are now described in setup.cfg
• Install-time requirements are now described in setup.cfg
• Removed some unused variables in setup.py
• The CI workflow has been cleaned up from duplicates, and build-time dependencies are not installed anymore as are already handled via pip in an isolated build environment.
• The CD workflow has NOT been updated yet to reflect the changes, as will be targeted during Uses cibuildwheel for releases #625