general: fix mypy errors after mypy and pytz stubs updates

see 968fd6d01d/stubs/pytz/pytz/tzinfo.pyi (L6)
it says all concrete instances should not be None

my/calendar/holidays.py

@@ -20,8 +20,8 @@ def _calendar():
     # TODO would be nice to do it dynamically depending on the past timezones...
     tz = LTZ._get_tz(datetime.now())
     assert tz is not None
-
-    code = zone_to_countrycode(tz.zone)
+    zone = tz.zone; assert zone is not None
+    code = zone_to_countrycode(zone)
     Cal = registry.get_calendars()[code]
     return Cal()