docs: add changelog entry and remove dateutil from __all__ variable

CHANGES.rst

@@ -6,7 +6,7 @@ Changelog
 
 Minor changes:
 
-- ...
+- add ``__all__`` variable to each modules in ``icalendar`` package
 
 Breaking changes:
 

src/icalendar/cal.py

@@ -767,5 +767,4 @@ component_factory = ComponentFactory()
 
 __all__ = ["Alarm", "Calendar", "Component", "ComponentFactory", "Event",
            "FreeBusy", "INLINE", "Journal", "Timezone", "TimezoneDaylight",
-           "TimezoneStandard", "Todo", "component_factory", "dateutil",
-           "types_factory"]
+           "TimezoneStandard", "Todo", "component_factory", "types_factory"]