Add type stubs file

[smessmer]

Add type stubs file (PEP484)

[pradyunsg]

I think it would be preferable to have inline comments instead. Also, xyz: str = None fails on newer mypy versions since the type should be Optional[str].

[cblegare]

Adding on top of what @pradyunsg said

According to https://www.python.org/dev/peps/pep-0561/

You also would have to add the py.typed marker, which boils to

• convert the appdirs.py to an appdirs folder with a __init__.py
• add a py.typed file in there
• make sure that file is bundled within your wheel with some setuptools incantation, i.e

# py_modules=["appdirs"], # no need for this anymore
package_data={"appdirs": ["py.typed"]},
packages=["appdirs"]

see also https://mypy.readthedocs.io/en/latest/installed_packages.html