schema: adds libyang.Enum to package

This patch adds missing Enum class to __init__.py to allow imports by
library users

Signed-off-by: Stefan Gula <steweg@gmail.com>

Author: steweg

libyang/__init__.py

@@ -67,6 +67,7 @@
 from .keyed_list import KeyedList
 from .log import configure_logging
 from .schema import (
+    Enum,
     Extension,
     ExtensionCompiled,
     ExtensionParsed,