www/py-django-mezzanine: Update to 3.1.10

• Update to 3.1.10
• Add LICENSE_FILE
• Update and sort RUN_DEPENDS
• Mezzanine now supports Python 3

• Update py-django-mezzanine-filebrowser to 0.3.5
• Update py-django-mezzanine-grappelli to 0.3.13
• Add LICENSE and LICENSE_FILE to both of the above

While I'm here:

• Sort USE_PYTHON values