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/api/serializers/organizer.py

@@ -1,7 +1,7 @@
 from decimal import Decimal
 
 from django.db.models import Q
-from django.utils.translation import get_language, ugettext_lazy as _
+from django.utils.translation import get_language, gettext_lazy as _
 from rest_framework import serializers
 from rest_framework.exceptions import ValidationError