[3.6] Mention enum as a potential use for metaclasses (GH-2982) (GH-3527)

(cherry picked from commit b8d1b9d7)

Doc/reference/datamodel.rst

@@ -1875,8 +1875,8 @@ Metaclass example
 ^^^^^^^^^^^^^^^^^
 
 The potential uses for metaclasses are boundless. Some ideas that have been
-explored include logging, interface checking, automatic delegation, automatic
-property creation, proxies, frameworks, and automatic resource
+explored include enum, logging, interface checking, automatic delegation,
+automatic property creation, proxies, frameworks, and automatic resource
 locking/synchronization.
 
 Here is an example of a metaclass that uses an :class:`collections.OrderedDict`