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/models/checkin.py

@@ -1,7 +1,7 @@
 from django.db import models
 from django.db.models import Exists, OuterRef
 from django.utils.timezone import now
-from django.utils.translation import pgettext_lazy, ugettext_lazy as _
+from django.utils.translation import gettext_lazy as _, pgettext_lazy
 from django_scopes import ScopedManager
 
 from pretix.base.models import LoggedModel