add a hook to autoupdate black in user configuration

[keewis]

As an alternative to #123, here's a hook to automate synchronizing
the black hook and the additional_dependencies entry of blackdoc.

I'm not sure if the way I implemented that hook (a regex) is
particularly reliable (besides the fact that multi-line regexes are
clearly not something I'm comfortable with), but using pyyaml for
this would strip any comments / reformat some entries, and
ruamel.yaml / ruyaml seem to struggle with maintenance (at least,
that's the impression I get).

• Closes Is it possible to explicitly set black version in pre-commit? #122
• Passes pre-commit run --all-files