[lit] Update lit.py shebang for Python3

Update shebang to always use Python3 when executing `lit.py` directly.

A previous change of mine [1] revealed that we still use Python2 on some
bot configurations that invoke `llvm/utils/lit/lit.py` as a script
directly (instead of `python3 path/to/lit.py`).

[1] https://reviews.llvm.org/D94734

Differential Revision: https://reviews.llvm.org/D95393