fix(icalendar): support icalendar v6

Enable backwards-compatible mode in icalendar v6 so that we can also
continue supporting older versions, which are still in use im
non-bleeding-edge distros.

src/autosuspend/checks/ical.py

@@ -16,6 +16,11 @@
 from ..util.datetime import is_aware, to_tz_unaware
 
 
+# Make v6 behave as if it were an older version to support a wider range.
+with suppress(AttributeError):
+    icalendar.use_pytz()
+
+
 @dataclass
 class CalendarEvent:
     summary: str