Upgrade to Django 3.0 and other dependencies (#1568)

* Upgrade Django to 3.0 and other dependencies to recent versions

* Fix otp version contsraint

* Remove six dependency

* Resolve some warnings

* Fix failing tests

* Update django-countries

* Resolve all RemovedInDjango31Warnings in test suite

* Run isort

* Fix import

* Update PostgreSQL version on travis

src/pretix/base/services/checkin.py

@@ -2,7 +2,7 @@ from django.db import transaction
 from django.db.models import Prefetch
 from django.dispatch import receiver
 from django.utils.timezone import now
-from django.utils.translation import ugettext as _
+from django.utils.translation import gettext as _
 
 from pretix.base.models import (
     Checkin, CheckinList, Order, OrderPosition, Question, QuestionOption,