zc.buildout: Add missing serializer safe global

None is safe global to be unserialized.

/reviewed-on nexedi/slapos.buildout!18

src/zc/buildout/buildout.py

@@ -91,6 +91,7 @@ class BuildoutSerialiser(object):
         'tuple': tuple,
         'False': False,
         'True': True,
+        'None': None,
     }}
 
     def loads(self, value):