[RFC][OpenMP] Update to Python3 for lit test

I think it's reasonable to upgrade to Python 3 for LIT test requirement because `lit` itself (`llvm/utils/lit/lit.py`) already switched to Python 3. In addition, LLVM already requires Python 3.6 to be the minimum version (https://llvm.org/docs/GettingStarted.html#software).

Reviewed By: jdoerfert, jhuber6

Differential Revision: https://reviews.llvm.org/D139855