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/webhooks.py

@@ -7,7 +7,7 @@ import requests
 from celery.exceptions import MaxRetriesExceededError
 from django.db.models import Exists, OuterRef, Q
 from django.dispatch import receiver
-from django.utils.translation import ugettext_lazy as _
+from django.utils.translation import gettext_lazy as _
 from django_scopes import scope, scopes_disabled
 from requests import RequestException