www/py-django21 (and dj21 ports): Deprecate / set expiration date

Django 2.1 reached its End-of-Life (EoL) on 2nd December. Deprecate the port
and its consumers (= www/py-dj21-*) accordingly.

Set the expiration date to the end of January 2020 to give users enough time
to migrate to Django 2.2+ because the EoL is already onset and the
deprecation comes quite late.

Reviewed by: koobs (python: maintainer)
Approved by: koobs (python: maintainer)
Differential Revision: https://reviews.freebsd.org/D22668