Don't proceed into setup.py too far if Python version is unsupported (#42870)
Summary:
This prevents confusing errors when the interpreter encounters some
syntax errors in the middle.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/42870
Reviewed By: albanD
Differential Revision: D23269265
Pulled By: ezyang
fbshipit-source-id: 61f62cbe294078ad4a909fa87aa93abd08c26344
