dateutil: Add comment explaining the use of ABCMeta (#8911)

stubs/python-dateutil/dateutil/tz/_common.pyi

@@ -9,6 +9,9 @@ class _DatetimeWithFold(datetime):
     @property
     def fold(self): ...
 
+# Doesn't actually have ABCMeta as the metaclass at runtime,
+# but mypy complains if we don't have it in the stub.
+# See discussion in #8908
 class _tzinfo(tzinfo, metaclass=abc.ABCMeta):
     def is_ambiguous(self, dt: datetime) -> bool: ...
     def fromutc(self, dt: datetime) -> datetime: ...