Update min python version to 3.7 in setup.py and mypy configs (#71494)
Summary:
As Python-3.6 have reached EOL
Pull Request resolved: https://github.com/pytorch/pytorch/pull/71494
Reviewed By: atalman
Differential Revision: D33667509
Pulled By: malfet
fbshipit-source-id: ab1f03085cfb9161df77ba5ce373b81f5e7ef3ae
(cherry picked from commit 60343166d97b1eb1649b29a78ad390d39926b642)
