Add css class hooks to BaseForm

[David Smith]

Form has a couple of hooks to add classes to a form. See ​docs.

However, the ​`BaseForm` does not have a class level initialisation of these. The classes are then later added via a hasattr check.

I was wondering if class level defaults (None) should be added for these to make them more discovarable, also the hasattr logic can just be replaced with Form.required_css_class

[Carlton Gibson]

Hey David, this is a duplicate of #14322.

Possibly that's worth re-opening (via a discussion, ​following triage guidelines for wontfix tickets) — auto-complete and type checkers much prefer attributes to be defined (as None?) rather than injected later.

I recall we've closed similar proposals along these lines though; often the references are able to remain entirely within the injecting code, whereas if they're declared (suddenly) they're split over multiple files. (e.g. excluding usages, required_css_class appears only in boundfield.py) — we didn't want to litter class definitions and/or init methods with None declarations. (Better the tooling gets cleverer.)

Also #14322 (and #3512) were concerned about backwards compatibility. The specific issue there may not apply after all this time, but it needs a thought.