Create some ports that are assigned to Django 3.1 instead Django 2.2

• Django 2.2 is currently the "default" version in the ports tree due its long term support until April 2022. Thus ports that are assigned to that version will exist until then.
  
  The following ports are required to make the upgrade of net-mgmt/netbox to the 2.9 release possible because it requires Django 3.1 and provides no backwards compatibility for previous versions of Django.
  • www/py-dj31-django-rq (copied from r539662 to use the 2.3.2 release for now)
  • www/py-dj31-django-auth-ldap
  • www/py-dj31-django-cacheops
  • www/py-dj31-django-cors-headers
  • www/py-dj31-django-debug-toolbar
  • www/py-dj21-django-filter
  • www/py-dj31-django-js-asset
  • www/py-dj31-django-mptt
  • www/py-dj31-django-prometheus
  • www/py-dj31-django-redis
  • www/py-dj31-django-tables2
  • www/py-dj31-django-taggit
  • www/py-dj31-django-timezone-field
  • www/py-dj31-djangorestframework
  • www/py-dj31-drf-yasg

• Also limit the new ports to Python 3.6+ due the requirements of Django 3.1 and add/update the related CONFLICTS_INSTALL entries.

Reviewed by: sunpoet
Differential Revision: https://reviews.freebsd.org/D27083