fix: install py.typed file so mypy will use included type defs

[bradenmacdonald]

I have rules==3.3 installed from pypi, but when I try to run type checks on a project that uses it, mypy reports this error:

my/project/rules.py:6: error: Skipping analyzing "rules": module is installed, but missing library stubs or py.typed marker  [import]

I believe the reason is that the py.typed file is not marked as package_data, so setuptools is not installing it. I think this should fix it.

[bradenmacdonald]

Note: I am not sure why the pypy build is failing, but it's unrelated to this PR - see discussion on other PR.

[federicobond]

Thank you! Rebased and merged in #181, commit is still attributed to you.