Upgrade for Django 3.x compatibility

[jacobcolyvan]

I found that this rep is causing errors with projects running Django 3.2.x, which look like the following:
TypeError: from_db_value() missing 1 required positional argument: 'context'.

Django has deprecated the context argument of Field.from_db_value() and Expression.convert_value(), as seen here: https://docs.djangoproject.com/en/3.0/releases/2.0/#features-deprecated-in-2-0.

This is solved by adding context=None to from_db_value(), seen here: https://github.com/ixc/python-edtf/blob/master/edtf/fields.py#L55.