Remove (most) Python 2 support from setup.py (#35617)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/35617
Python 2 has reached end-of-life and is no longer supported by PyTorch.
Now we can clean up some cruft that we put in place to support it.
Test Plan: CI
Differential Revision: D20842883
Pulled By: dreiss
fbshipit-source-id: 18dc5219ba99658c0ca7e2f26863df008c420e6a
