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/multidomain/models.py

@@ -1,6 +1,6 @@
 from django.core.cache import cache
 from django.db import models
-from django.utils.translation import ugettext_lazy as _
+from django.utils.translation import gettext_lazy as _
 from django_scopes import scopes_disabled
 
 from pretix.base.models import Event, Organizer

src/pretix/multidomain/templatetags/eventurl.py

@@ -3,7 +3,7 @@ from django.template import TemplateSyntaxError
 from django.template.base import kwarg_re
 from django.template.defaulttags import URLNode
 from django.urls import NoReverseMatch
-from django.utils.encoding import smart_text
+from django.utils.encoding import smart_str
 from django.utils.html import conditional_escape
 
 from pretix.multidomain.urlreverse import build_absolute_uri
@@ -20,7 +20,7 @@ class EventURLNode(URLNode):
     def render(self, context):
         from pretix.multidomain.urlreverse import eventreverse
         kwargs = {
-            smart_text(k, 'ascii'): v.resolve(context)
+            smart_str(k, 'ascii'): v.resolve(context)
             for k, v in self.kwargs.items()
         }
         view_name = self.view_name.resolve(context)