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/admin/installation/manual_smallscale.rst

@@ -12,7 +12,7 @@ solution with many things readily set-up, look at :ref:`dockersmallscale`.
              get it right. If you're not feeling comfortable managing a Linux server, check out our hosting and service
              offers at `pretix.eu`_.
 
-We tested this guide on the Linux distribution **Debian 8.0** but it should work very similar on other
+We tested this guide on the Linux distribution **Debian 10.0** but it should work very similar on other
 modern distributions, especially on all systemd-based ones.
 
 Requirements
@@ -133,7 +133,7 @@ command if you're running MySQL::
 
     (venv)$ pip3 install "pretix[postgres]" gunicorn
 
-Note that you need Python 3.5 or newer. You can find out your Python version using ``python -V``.
+Note that you need Python 3.6 or newer. You can find out your Python version using ``python -V``.
 
 We also need to create a data directory::
 

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)