enable type hints when importing nox

[cs01]

Because nox is not marked as containing type hints, any libraries that imports nox will get a mypy error saying the library is untyped. To fix this, users have had to do

import nox  # type: ignore

to avoid errors such as

error: Cannot find implementation or library stub for module named 'nox'
note: See https://mypy.readthedocs.io/en/latest/running_mypy.html#missing-imports

This PR implements instructions in the mypy documentation to tell mypy that nox does indeed have inline types (https://mypy.readthedocs.io/en/latest/installed_packages.html#making-pep-561-compatible-packages). Though rather strange and arbitrary, this is the standard way to do this. GvR himself committed a py.typed to mypy here.

If this code is merged, the # type: ignore comment will no longer be necessary for users of nox, and they can be confident their code doesn't have type errors.

[theacodes]

Thanks, @cs01!