ngclient: Fix trusted set iterator

We're unlikely to use the iterator but it's required to implement Mapping.

Signed-off-by: Jussi Kukkonen <jkukkonen@vmware.com>

Author: Jussi Kukkonen

tuf/ngclient/_internal/trusted_metadata_set.py

@@ -116,7 +116,7 @@ def __len__(self) -> int:
 
     def __iter__(self) -> Iterator[Metadata]:
         """Returns iterator over all Metadata objects in TrustedMetadataSet"""
-        return iter(self._trusted_set)
+        return iter(self._trusted_set.values())
 
     # Helper properties for top level metadata
     @property