Remove python2 and 3.5 from requirements.txt, README and docs (#35677)
Summary:
Some more cleanup now that we no longer support python2 or 3.5 on master and eventually PyTorch 1.6 release.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/35677
Differential Revision: D20838097
Pulled By: orionr
fbshipit-source-id: 95d553a1e8769f3baa395e0bc6d4ce7cd93236e9
