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/presale/views/locale.py

@@ -2,7 +2,7 @@ from datetime import datetime, timedelta
 
 from django.conf import settings
 from django.http import HttpResponseRedirect
-from django.utils.http import is_safe_url
+from django.utils.http import url_has_allowed_host_and_scheme
 from django.views.generic import View
 
 from pretix.helpers.cookies import set_cookie_without_samesite
@@ -14,7 +14,7 @@ class LocaleSet(NoSearchIndexViewMixin, View):
 
     def get(self, request, *args, **kwargs):
         url = request.GET.get('next', request.headers.get('Referer', '/'))
-        url = url if is_safe_url(url, allowed_hosts=[request.get_host()]) else '/'
+        url = url if url_has_allowed_host_and_scheme(url, allowed_hosts=[request.get_host()]) else '/'
         resp = HttpResponseRedirect(url)
 
         locale = request.GET.get('locale')