Update tuf/api/metadata.py (WIP)

Author: lukpueh

tuf/api/metadata.py

@@ -308,7 +308,7 @@ def __init__(
 
     # Deserialization (factories).
     @classmethod
-    def from_dict(cls, signed_dict) -> 'Signed':
+    def from_dict(cls, signed_dict: JsonDict) -> 'Signed':
         """Creates Signed object from its JSON/dict representation. """
 
         # Convert 'expires' TUF metadata string to a datetime object, which is