[lit] Remove python 2.7 code paths in builtin diff
Lit's builtin diff command had some Python 2.7 code paths lying around
that we can probably get rid of at this point. LLVM at this point
requires Python 3.8 at minimum. Keeping lit working at a lower version
is a reasonable goal, but I think we can probably drop python 2 support
at this point given how long it has been deprecated and how long LLVM
has supported Python 3.
Reviewers: jdenny-ornl, ilovepi, petrhosek
Reviewed By: ilovepi
Pull Request: https://github.com/llvm/llvm-project/pull/157558
