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/sendmail/__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/sendmail/forms.py

@@ -1,6 +1,6 @@
 from django import forms
 from django.urls import reverse
-from django.utils.translation import pgettext_lazy, ugettext_lazy as _
+from django.utils.translation import gettext_lazy as _, pgettext_lazy
 from django_scopes.forms import SafeModelMultipleChoiceField
 from i18nfield.forms import I18nFormField, I18nTextarea, I18nTextInput
 

src/pretix/plugins/sendmail/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 logentry_display
 from pretix.control.signals import nav_event

src/pretix/plugins/sendmail/views.py

@@ -6,7 +6,7 @@ from django.db.models import Exists, OuterRef, Q
 from django.http import Http404
 from django.shortcuts import redirect
 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.generic import FormView, ListView
 
 from pretix.base.email import get_available_placeholders