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/checkinlists/__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/checkinlists/exporters.py

@@ -10,7 +10,7 @@ from django.db.models.functions import Coalesce, NullIf
 from django.urls import reverse
 from django.utils.formats import date_format
 from django.utils.timezone import is_aware, make_aware
-from django.utils.translation import pgettext, ugettext as _, ugettext_lazy
+from django.utils.translation import gettext as _, gettext_lazy, pgettext
 from jsonfallback.functions import JSONExtract
 from pytz import UTC
 from reportlab.lib.units import mm
@@ -180,7 +180,7 @@ class TableTextRotate(Flowable):
 class PDFCheckinList(ReportlabExportMixin, CheckInListMixin, BaseExporter):
     name = "overview"
     identifier = 'checkinlistpdf'
-    verbose_name = ugettext_lazy('Check-in list (PDF)')
+    verbose_name = gettext_lazy('Check-in list (PDF)')
 
     @property
     def export_form_fields(self):
@@ -312,7 +312,7 @@ class PDFCheckinList(ReportlabExportMixin, CheckInListMixin, BaseExporter):
 class CSVCheckinList(CheckInListMixin, ListExporter):
     name = "overview"
     identifier = 'checkinlist'
-    verbose_name = ugettext_lazy('Check-in list')
+    verbose_name = gettext_lazy('Check-in list')
 
     @property
     def additional_form_fields(self):