Issue #14448: mention pytz; patch by Andrew Svetlov

Doc/library/datetime.rst

@@ -1521,6 +1521,14 @@ Applications that can't bear such ambiguities should avoid using hybrid
 other fixed-offset :class:`tzinfo` subclass (such as a class representing only
 EST (fixed offset -5 hours), or only EDT (fixed offset -4 hours)).
 
+.. seealso::
+
+   `pytz <http://pypi.python.org/pypi/pytz/>`_
+      The Standard Library has no :class:`tzinfo` instances except for UTC, but
+      it exists a third-party library which brings Olson timezone database to
+      Python: `pytz`.
+
+      `pytz` contains up-to-date information and its usage is recommended.
 
 .. _strftime-strptime-behavior: