Introduce black formatter

[no2chem]

This PR introduces the black formatter as a pre-commit hook.

When you run git commit, black automatically re-formats the code before passing it to flake8.

example:

vscode ➜ /workspaces/wideq (format ✗) $ git commit -a -m "introduce black formatter"
black....................................................................Passed
Flake8...................................................................Passed

There are some differences in how black and flake8 interpret pep. I'm not really opinionated here as I am not a python expert, but it seems that the black authors think they are right in interpreting the PEP, and black fixes the formatting in a consistent way that an explicit rule shouldn't be required. The exceptions are defined in the .flake8 file.

This change should make it easier to compare changes, as small style differences should be make consistent by the tool. For example, #132 has a lot of changes because I ran black just to correct the flake8 errors. This eliminates formatting differences from pull requests.

You can always skip the precommit hooks by running

git commit --no-verify

[no2chem]

well, it seems to really not want to run automatically on the test directory, not sure why...

[sampsyo]

Hi there—although I'm not a frequent Black user, I'm OK with adding it if users are excited about automatic formatting. (TBH, I find gentle feedback from flake8 enough to ensure consistency, although I know some folks really get a lot of value out of automatic reformatters.)

If we're going to do this, however, I think it would be best to enforce its use "server-side" rather than relying on folks to use the pre-commit hook. That would mean reconfiguring our GitHub actions to run something like black --check or black-primer in addition to flake8 and mypy. Would anybody be interested in taking care of that?