Clarify Python 3.5 is the minimum supported version in the installation section. (#41937)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/41937
Reviewed By: izdeby
Differential Revision: D22702924
Pulled By: mrshenli
fbshipit-source-id: 67306435e80f80236b585f1d5406444daec782d6
