Format and check all Python code using python black

[wohali]

Inspired by our formatting work on Elixir, this PR proposes doing the same thing for our Python code.

Python Black is an opinionated code formatter now adopted by prominent Python projects such as pip. It is easy to use and provides both formatting and check modes.

The python-black target builds a python3 venv at .venv and installs black if possible. Since black is Python 3.6 and up only, we skip the check on systems with an older Python 3.x and print a warning. Our CI systems will still catch problems from people who submit PRs who don't have 3.6+ since CI is configured to run this check automatically on at least one platform (e.g., Jenkins Ubuntu Bionic, or Python 3.6 explicitly requested under Travis CI.)

Assuming the check runs, it fails if any file would be changed by the formatter.

A Makefile target is provided that actually does the updating (make python-black-update, it simply omits the --check flag) for happy dev fingers to fix up python files.

The change to the docs dependency in rebar.config.script is necessary because of embedded Python files in that repo that have only just been re-formatted. Once docs is tagged with 2.3.0, this dep will be reverted to a tag-type reference, not a commit.

@nickva I had to change the location of the Mango venv to src/mango/.venv so it would be automatically ignored by the formatter, otherwise it tries to format the entire venv. Oops :)

Testing recommendations

If you have Python 3.6+, try make python-black and see if it works.

if you don't, try the same command and make sure it is a noop (with an exit code of 0).

Checklist

• Code is written and works correctly;
• Changes are covered by tests;
• Documentation reflects the changes;

[wohali]

Any comments from @janl @davisp @iilyak @eiri or anyone else who does Python-y things? Waiting on a +1 or comments to the contrary.

[davisp]

+1 Yay consistency!