Add a make target to run the black formatter
Add a make target to run the black formatter

Add a `black` target that runs `black --check` on Python files, and
run it in the `make` target. If any files would be reformatted, the
build fails.

Change-Id: I8f7e94d3c67a15643b2b2ba07d94301f2aab3526

dpursehouse/pygerrit2