Add blacken-docs and pycln pre-commit hooks

[Skylion007]

Description

• Applies blacken-docs and therefore black code formatting to the RST documentation.
• Applies pycln and removes unused imports.
• Fixes syntax highlighting blocks that were improperly set.
• I added the associated pre-commit hooks as well.

Suggested changelog entry:

* Add blacken-docs pre-commit hook
* Add pycln pre-commit hook and remove unused imports 
* Fix highlighting hints and errors in RST docs

[henryiii]

I'd be fine to add the pinning. Black doesn't really change that fast (or practically at all), so a slightly older docs format is unlikely to clash with the main format. We'd get auto fixes, for example,w with that.

[Skylion007]

@henryiii Okay, I'll add it.

[Skylion007]

@henryiii Yeah, I would feel even better though if someone could add a CI check that checks that version match in the pre-commit.yaml though like we do for other version checks.

[henryiii]

I wasn't aware of pycln, looks useful. :)

[Skylion007]

@henryiii @rwgk Okay, this PR is ready to go unless anyone has objections?