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/views/oauth.py

@@ -2,7 +2,7 @@ import logging
 
 from django import forms
 from django.conf import settings
-from django.utils.translation import ugettext as _
+from django.utils.translation import gettext as _
 from oauth2_provider.exceptions import OAuthToolkitError
 from oauth2_provider.forms import AllowForm
 from oauth2_provider.views import (

src/pretix/api/views/order.py

@@ -9,7 +9,7 @@ from django.db.models.functions import Coalesce, Concat
 from django.http import FileResponse, HttpResponse
 from django.shortcuts import get_object_or_404
 from django.utils.timezone import make_aware, now
-from django.utils.translation import ugettext as _
+from django.utils.translation import gettext as _
 from django_filters.rest_framework import DjangoFilterBackend, FilterSet
 from django_scopes import scopes_disabled
 from rest_framework import mixins, serializers, status, viewsets