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/plugins/statistics/__init__.py

@@ -1,5 +1,5 @@
 from django.apps import AppConfig
-from django.utils.translation import ugettext_lazy as _
+from django.utils.translation import gettext_lazy as _
 
 from pretix import __version__ as version
 

src/pretix/plugins/statistics/signals.py

@@ -1,6 +1,6 @@
 from django.dispatch import receiver
 from django.urls import resolve, reverse
-from django.utils.translation import ugettext_lazy as _
+from django.utils.translation import gettext_lazy as _
 
 from pretix.base.signals import order_paid, order_placed
 from pretix.control.signals import nav_event