Include 'enum' module as warnings indirectly imports it since python 3.6

Fixes #962

Related CPython change: http://bugs.python.org/issue28082

Author: yan12125

virtualenv.py

@@ -155,6 +155,8 @@ def get_installed_pythons():
             '_collections_abc',
             '_bootlocale',
         ])
+    if minver >= 6:
+        REQUIRED_MODULES.extend(['enum'])
 
 if is_pypy:
     # these are needed to correctly display the exceptions that may happen