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/base/migrations/0077_auto_20171124_1629.py

@@ -6,7 +6,7 @@ import django.core.validators
 import django.db.models.deletion
 from django.conf import settings
 from django.db import migrations, models
-from django.utils.translation import ugettext as _
+from django.utils.translation import gettext as _
 
 import pretix.base.validators
 from pretix.base.i18n import language

src/pretix/base/migrations/0077_auto_20171124_1629_squashed_0088_auto_20180328_1217.py

@@ -7,7 +7,7 @@ from django.db import migrations, models
 from django.db.models import F
 from django.db.models.functions import Concat
 from django.utils.crypto import get_random_string
-from django.utils.translation import ugettext as _
+from django.utils.translation import gettext as _
 
 import pretix.base.models.auth
 import pretix.base.validators