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/helpers/daterange.py

@@ -1,5 +1,5 @@
 from django.template.defaultfilters import date as _date
-from django.utils.translation import get_language, ugettext_lazy as _
+from django.utils.translation import get_language, gettext_lazy as _
 
 
 def daterange(df, dt):

src/pretix/helpers/dicts.py

@@ -7,3 +7,8 @@ def merge_dicts(*dict_args):
     for dictionary in dict_args:
         result.update(dictionary)
     return result
+
+
+def move_to_end(d, k):
+    v = d.pop(k)
+    d[k] = v

src/pretix/helpers/escapejson.py

@@ -1,5 +1,4 @@
-from django.utils import six
-from django.utils.encoding import force_text
+from django.utils.encoding import force_str
 from django.utils.functional import keep_lazy
 from django.utils.safestring import SafeText, mark_safe
 
@@ -19,13 +18,13 @@ _json_escapes_attr = {
 }
 
 
-@keep_lazy(six.text_type, SafeText)
+@keep_lazy(str, SafeText)
 def escapejson(value):
     """Hex encodes characters for use in a application/json type script."""
-    return mark_safe(force_text(value).translate(_json_escapes))
+    return mark_safe(force_str(value).translate(_json_escapes))
 
 
-@keep_lazy(six.text_type, SafeText)
+@keep_lazy(str, SafeText)
 def escapejson_attr(value):
     """Hex encodes characters for use in a html attributw script."""
-    return mark_safe(force_text(value).translate(_json_escapes_attr))
+    return mark_safe(force_str(value).translate(_json_escapes_attr))