Support Django 3+

[jspetrak]

Upgrade to Django 3.1 now because django.db.models.JSONField is used by Django to store JSON into DB native format. (issue #101)

We intend to upgrade multigtfs to 3.1 as soon as posible, and when 3.2 LTS is released, move to Django 3.2 LTS and keep it as minimal supported version as long as it is viable.