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/presale/checkoutflow.py

@@ -10,7 +10,7 @@ from django.shortcuts import redirect
 from django.utils import translation
 from django.utils.functional import cached_property
 from django.utils.translation import (
-    get_language, pgettext_lazy, ugettext_lazy as _,
+    get_language, gettext_lazy as _, pgettext_lazy,
 )
 from django.views.generic.base import TemplateResponseMixin
 from django_scopes import scopes_disabled