Thanks Tim Graham and Loïc Bistuer for the reviews.

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

This partially reverts 983c158d

Generated correct admindocs URLs on Python 3. URLs generate 404s on
Python 2, as in older versions of Django.

Thanks Andrew Nester, Berker Peksag, and Tim Graham for reviews.

The current board has been re-elected.