Prefer python3 in clang_format (#35490)
Summary:
On most Linux distros `python` still points to python-2.x
Pull Request resolved: https://github.com/pytorch/pytorch/pull/35490
Differential Revision: D20676691
Pulled By: malfet
fbshipit-source-id: 0d4519b83cfebb108edc0628bf036a541247584e
