[NFC][Py Reformat] Reformat python files in llvm

This is the first commit in a series that will reformat
all the python files in the LLVM repository.

Reformatting is done with `black`.

See more information here:

https://discourse.llvm.org/t/rfc-document-and-standardize-python-code-style

Reviewed By: jhenderson, JDevlieghere, MatzeB

Differential Revision: https://reviews.llvm.org/D150545