API: Add timezone attribute to events

Note: I still believe the issues described in https://github.com/pretix/pretix/issues/1378
are a problem, and I'm still not keen on adding settings properties to
the API until we have a proper design for it. However, I'm making an
exception here since the list of events can't be used in a very useful
way with not access to the timezone.

src/tests/api/conftest.py

@@ -38,6 +38,7 @@ def event(organizer, meta_prop):
         is_public=True
     )
     e.meta_values.create(property=meta_prop, value="Conference")
+    e.settings.timezone = 'Europe/Berlin'
     return e