www/py-django-cms: Update to 3.7.1

• Also assign the port to Django 2.2 because Django 1.11 is End-of-Life since April. [1]

• Do the same for its dependencies (including www/py-django-classy-tags [2]) and bump PORTREVISION accordingly.

Changelog:

https://github.com/divio/django-cms/blob/3.7.1/CHANGELOG.rst

PR: 245361 [1] 245360 [2]
Approved by: maintainer timeout (14 days) [1] [2]