Thanks Ola Sitarska for the initial patch.

Fixed #26998 -- Reverted some admin checks from checking field.many_to_many back to isinstance(field, models.ManyToManyField).

This partially reverts 983c158d

Thanks Markus Holtermann, Collin Anderson and Tim Graham for the reviews.

Refs #22792

This allows dynamically-generated attributes to be specified in
checked ModelAdmin attributes without triggering errors.

ModelAdmin added readonly_fields to exclude, but would not undeclare
them if they were overridden.

Thanks to Collin Anderson for the review and suggestions and Tim for the
final review.

Thanks chrj for the suggestion.

Thanks Stephen Burrows for work on the patch as well.

Forwardport of 2914f669 from stable/1.7.x