backends/zoneinfo.py: Skip using the "Factory" timezone

Referencing the "Factory" timezone can lead to ValueError exceptions on some systems, e.g. FreeBSD, if the system zoneinfo database is used:

ValueError: Invalid STD format in b'<Local time zone must be set--use tzsetup>0'

Thus drop the "Factory" timezone from the sets before iterating over them to avoid the aforementioned issue.

timezone_field/backends/zoneinfo.py

@@ -10,6 +10,10 @@ class ZoneInfoBackend(TimeZoneBackend):
     utc_tzobj = zoneinfo.ZoneInfo("UTC")
     all_tzstrs = zoneinfo.available_timezones()
     base_tzstrs = zoneinfo.available_timezones()
+    # Remove the "Factory" timezone as it can cause ValueError exceptions on
+    # some systems, e.g. FreeBSD, if the system zoneinfo database is used.
+    all_tzstrs.discard("Factory")
+    base_tzstrs.discard("Factory")
 
     def is_tzobj(self, value):
         return isinstance(value, zoneinfo.ZoneInfo)