Revert "First steps into pytz deprecation"

This reverts commit e4e7d50659.

src/pretix/base/timeframes.py

@@ -20,10 +20,11 @@
 # <https://www.gnu.org/licenses/>.
 #
 import calendar
-from datetime import date, datetime, time, timedelta, timezone
+from datetime import date, datetime, time, timedelta
 from itertools import groupby
 from typing import Optional, Tuple
 
+import pytz
 from django import forms
 from django.core.exceptions import ValidationError
 from django.utils.formats import date_format
@@ -389,7 +390,7 @@ class SerializerDateFrameField(serializers.CharField):
         if data is None:
             return None
         try:
-            resolve_timeframe_to_dates_inclusive(now(), data, timezone.utc)
+            resolve_timeframe_to_dates_inclusive(now(), data, pytz.UTC)
         except:
             raise ValidationError("Invalid date frame")