Time machine mode [Z#23129725] (#3961)

Allows organizers to test their shop as if it were a different date and time.

Implemented using a time_machine_now() function which is used instead of regular now(), which can overlay the real date time with a value from a ContextVar, assigned from a session value in EventMiddleware.

For more information, see doc/development/implementation/timemachine.rst

---------

Co-authored-by: Richard Schreiber <schreiber@rami.io>
Co-authored-by: Raphael Michel <michel@rami.io>

src/pretix/base/services/memberships.py

@@ -25,13 +25,13 @@ from typing import List, Optional
 from dateutil.relativedelta import relativedelta
 from django.core.exceptions import ValidationError
 from django.utils.formats import date_format
-from django.utils.timezone import now
 from django.utils.translation import gettext_lazy as _
 
 from pretix.base.models import (
     AbstractPosition, CartPosition, Customer, Event, Item, Membership, Order,
     OrderPosition, SubEvent,
 )
+from pretix.base.timemachine import time_machine_now
 from pretix.helpers import OF_SELF
 
 
@@ -48,7 +48,7 @@ def membership_validity(item: Item, subevent: Optional[SubEvent], event: Event):
 
     else:
         # Always start at start of day
-        date_start = now().astimezone(tz).replace(hour=0, minute=0, second=0, microsecond=0)
+        date_start = time_machine_now().astimezone(tz).replace(hour=0, minute=0, second=0, microsecond=0)
         date_end = date_start
 
         if item.grant_membership_duration_months: