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/paypal/__init__.py

@@ -1,6 +1,6 @@
 from django.apps import AppConfig
 from django.utils.functional import cached_property
-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/paypal/payment.py

@@ -10,7 +10,7 @@ from django.core import signing
 from django.http import HttpRequest
 from django.template.loader import get_template
 from django.urls import reverse
-from django.utils.translation import ugettext as __, ugettext_lazy as _
+from django.utils.translation import gettext as __, gettext_lazy as _
 from i18nfield.strings import LazyI18nString
 from paypalrestsdk.exceptions import BadRequest
 from paypalrestsdk.openid_connect import Tokeninfo

src/pretix/plugins/paypal/signals.py

@@ -4,7 +4,7 @@ from collections import OrderedDict
 from django import forms
 from django.dispatch import receiver
 from django.template.loader import get_template
-from django.utils.translation import ugettext_lazy as _
+from django.utils.translation import gettext_lazy as _
 
 from pretix.base.signals import (
     logentry_display, register_global_settings, register_payment_providers,

src/pretix/plugins/paypal/views.py

@@ -9,7 +9,7 @@ from django.db.models import Sum
 from django.http import HttpResponse, HttpResponseBadRequest
 from django.shortcuts import get_object_or_404, redirect, render
 from django.urls import reverse
-from django.utils.translation import ugettext_lazy as _
+from django.utils.translation import gettext_lazy as _
 from django.views.decorators.clickjacking import xframe_options_exempt
 from django.views.decorators.csrf import csrf_exempt
 from django.views.decorators.http import require_POST