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/pretixdroid/__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/pretixdroid/forms.py

@@ -1,6 +1,6 @@
 from django import forms
 from django.urls import reverse
-from django.utils.translation import ugettext_lazy as _
+from django.utils.translation import gettext_lazy as _
 from django_scopes.forms import (
     SafeModelChoiceField, SafeModelMultipleChoiceField,
 )

src/pretix/plugins/pretixdroid/models.py

@@ -2,7 +2,7 @@ import string
 
 from django.db import models
 from django.utils.crypto import get_random_string
-from django.utils.translation import ugettext_lazy as _
+from django.utils.translation import gettext_lazy as _
 
 
 class AppConfiguration(models.Model):

src/pretix/plugins/pretixdroid/signals.py

@@ -1,7 +1,7 @@
 
 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 logentry_display
 from pretix.control.logdisplay import _display_checkin

src/pretix/plugins/pretixdroid/views.py

@@ -14,7 +14,7 @@ from django.urls import reverse
 from django.utils.decorators import method_decorator
 from django.utils.functional import cached_property
 from django.utils.timezone import now
-from django.utils.translation import ugettext_lazy as _
+from django.utils.translation import gettext_lazy as _
 from django.views.decorators.csrf import csrf_exempt
 from django.views.generic import TemplateView, View
 from django_scopes import scope, scopes_disabled