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/control/forms/item.py

@@ -7,7 +7,7 @@ from django.db.models import Max
 from django.forms.formsets import DELETION_FIELD_NAME
 from django.urls import reverse
 from django.utils.translation import (
-    pgettext_lazy, ugettext as __, ugettext_lazy as _,
+    gettext as __, gettext_lazy as _, pgettext_lazy,
 )
 from django_scopes.forms import (
     SafeModelChoiceField, SafeModelMultipleChoiceField,