PEP 681: Rename transform_descriptor_types to delete_class_attributes

[debonte]

We decided to rename transform_descriptor_types to delete_class_attributes because:

• Descriptors only work as expected when stored in class attributes, so we needed to clarify that the class attribute was retained (not deleted) for descriptor fields.
• Some libraries retain class attributes for all fields.

Renamed the parameter and added a new section describing the behavior in more detail.

Also removed the line about Django limitations making dataclass_transform usage impractical, since we've gotten user feedback that that's not the case. I can create a separate PR for this if preferred.

cc: @carljm @erictraut

[hugovk]

Code formatting on True / False:

[JelleZijlstra]

Looks good apart from one nit.

Should I merge this? After we merge it, I can provide a new typing-extensions release with the new behavior.

[carljm]

I regret holding things up when everyone else seems satisfied, but I really don't think delete_class_attributes is an accurate description of the behavior being specified here, and I think it would be a mistake to submit the PEP with this name.

Also, after exploring the current runtime behavior of @dataclass a bit more, I have my doubts about whether this even makes sense as an added feature for the PEP.

I'll elaborate on the existing mailing list thread, since that's where the design discussion has taken place so far.

[debonte]

Abandoning this PR in favor of #2477