backport the typeshed contextlib.pyi file and add a py.typed

[graingert]

No description provided.

[ncoghlan]

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

• convert distribution structure from contextlib2.py to contextlib2/__init__.py
• add contextlib2/py.typed
• copy the typeshed contextlib.pyi file in as contextlib2/__init__.pyi

[graingert]

You could alternatively maintain a types-contextlib2 package with the pyi file in

[ncoghlan]

True, but the pyi file isn't that big, and having to keep a second package in sync would be annoying.

[ncoghlan]

It turned out the other concrete benefit to shipping a natively typed package was being able to integrate a self-consistency check into the module CI: #37