Modernize packaging a bit
Adding pyproject.toml allows the package to be installed using PEP517 instead of legacy setup.py install method. At least pip says so in a warning when installing without the wheel package available.
Change setup.py to setup.cfg (using a script[0]) because setup.py is out of fashion these days.
Remove MANIFEST.ini since LICENSE is implied[1].
[0] https://github.com/gvalkov/setuptools-py2cfg [1] https://setuptools.pypa.io/en/latest/references/keywords.html#keyword-license-files
Changed from setup.cfg to pyproject.toml, seems to work. Added it in a separate commit just in case.
~~One final change can be done to eliminate the tool.setuptools py-modules section by moving from a single module mypy_extensions.py to a package, then setuptools can auto-discover it.~~ Actually seems to work with the module as well, so just removed it.