PEP 484: Add @type_check_only (#858)

Per discussion in python/typing#597.

Author: srittau

pep-0484.txt

@@ -2047,6 +2047,10 @@ Convenience definitions:
 * @no_type_check_decorator, a decorator to create your own decorators
   with the same meaning as ``@no_type_check`` (see below)
 
+* @type_check_only, a decorator only available during type checking
+  for use in stub files (see above); marks a class or function as
+  unavailable during runtime
+
 * @overload, described earlier
 
 * get_type_hints(), a utility function to retrieve the type hints from a