www/py-djangorestframework: Update to 3.11.0

• Switch the port to Django 2.2 (current LTS release) as well because Django 1.11 is End-of-Life since April. Do the same for www/py-drf-yasg.

• Repo-copy www/py-djangorestframework to www/py-djangorestframework311 which will be used as a transitional package by www/seahub until it gets support for Django 2.2+ in the next weeks/months.

• Drop the Python 2.7 support of the remaining consumers to avoid breakage of "bulk -a".

• Mark both repo-copied ports www/py-dj22-{djangorestframework,drf-yasg} as deprecated because they're obsolete now.

PR: 245309
Reviewed by: sunpoet (earlier revision), ultima
Differential Revision: https://reviews.freebsd.org/D24850