Fix bug in oci/layer.py __iter__() not returning Iterator

Author: ByteOtter

src/gardenlinux/oci/layer.py

@@ -104,7 +104,7 @@ def __iter__(self):
         :since:  0.7.0
         """
 
-        iter(_SUPPORTED_MAPPING_KEYS)
+        return iter(_SUPPORTED_MAPPING_KEYS)
 
     def __len__(self):
         """