Add PEP-561 Support by adding a py.typed file

[lewis6991]

I'm trying to run the latest mypy on a sphinx extension I am writing and mypy is giving the error:

error: Cannot find module named 'sphinx'

However looking at the source code for a lot of sphinx, it appears much of the code includes type annotations. The mypy docs state that if source code includes annotations in the source then a package can be made PEP-561 compatible by adding a py.typed file.

I tried this myself by adding a py.typed file to the sphinx directory in my site-packages and now mypy is happy.

This may be related to #4824

[tk0miya]

Could you send a pull request please? Then I'll ship it in next release.

Note: 2.0 branch is good for this.