clang-format-diff: Make it work with python3 too

Summary: It is not necessary, but would be nice if the script run on python3 as well (as opposed to only python2, which is going to be deprecated https://pythonclock.org/)

Contributed by MarcoFalke!

Reviewers: krasimir

Reviewed By: krasimir

Subscribers: lebedev.ri, sammccall, cfe-commits

Differential Revision: https://reviews.llvm.org/D48098