[BE] Fix mypy local run on MacOS (#69097)
Summary:
Unversioned python invocations should not be used, as it can be aliased to Python-2
Also invoke mypy as `python3 -mmypy` as binary aliases are not always available for user installation
Pull Request resolved: https://github.com/pytorch/pytorch/pull/69097
Reviewed By: janeyx99
Differential Revision: D32729367
Pulled By: malfet
fbshipit-source-id: 7539bd0af15f97eecddfb142dba7de7f3587083d
