Run "black" to reformat all our python
"Black" is an "opinionated" python formatter, whose opinionatedness is somewhat in the spirit of rustfmt.
This MR runs black with default settings on all of our python code in Arti. It was produced by the following commands
# Everything in python/
black python/
# Everything with a .py extension
fd '.py$' -X black
# Everything with a python shebang.
git grep -l '#! */usr/bin/env *python' | xargs black