Add python_requires to help pip (#23863)
Summary:
`python_requires` helps the installer choose the correct version of this package for the user's running Python.
This is especially necessary when dropping Python 2 (https://github.com/pytorch/pytorch/issues/23795) but is useful now too.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/23863
Differential Revision: D16692908
Pulled By: soumith
fbshipit-source-id: 3c9ba2eb1d1cf12763d6284daa4f18f605abb373
