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

doc/development/api/customview.rst

@@ -66,7 +66,7 @@ event-related views, there is also a signal that allows you to add the view to t
 
     from django.urls import resolve, reverse
     from django.dispatch import receiver
-    from django.utils.translation import ugettext_lazy as _
+    from django.utils.translation import gettext_lazy as _
     from pretix.control.signals import nav_event
 
 

doc/development/api/plugins.rst

@@ -61,7 +61,7 @@ A working example would be::
         from pretix.base.plugins import PluginConfig
     except ImportError:
         raise RuntimeError("Please use pretix 2.7 or above to run this plugin!")
-    from django.utils.translation import ugettext_lazy as _
+    from django.utils.translation import gettext_lazy as _
 
 
     class PaypalApp(PluginConfig):

doc/development/implementation/logging.rst

@@ -69,7 +69,7 @@ We now need a way to translate the action codes like ``pretix.event.changed`` in
 strings. The :py:attr:`pretix.base.signals.logentry_display` signals allows you to do so. A simple
 implementation could look like::
 
-    from django.utils.translation import ugettext as _
+    from django.utils.translation import gettext as _
     from pretix.base.signals import logentry_display
 
     @receiver(signal=logentry_display)