Fail setup.py if trying to set up with Python 2 (#35613)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/35613
Python 2 has reached end-of-life and is no longer supported by PyTorch.
To spare users from a long, doomed setup when trying to use PyTorch with
Python 2, detect this case early and fail with a clear message. This
commit covers setup.py.
Test Plan: Attempted to build PyTorch with Python 2 and saw a clear error *quickly*.
Differential Revision: D20842881
Pulled By: dreiss
fbshipit-source-id: caaaa0dbff83145ff668bd25df6d7d4b3ce12e47
