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/users.py

@@ -4,7 +4,7 @@ from django.contrib.auth.password_validation import (
     password_validators_help_texts, validate_password,
 )
 from django.db.models import Q
-from django.utils.translation import ugettext_lazy as _
+from django.utils.translation import gettext_lazy as _
 from pytz import common_timezones
 
 from pretix.base.models import User