Metadata API: Fix DelegatedRole.from_dict() return type

Also mark the argument as Dict as we will pop() it.

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

Author: Jussi Kukkonen

tuf/api/metadata.py

@@ -815,7 +815,7 @@ def __init__(
         self.path_hash_prefixes = path_hash_prefixes
 
     @classmethod
-    def from_dict(cls, role_dict: Mapping[str, Any]) -> "Role":
+    def from_dict(cls, role_dict: Dict[str, Any]) -> "DelegatedRole":
         """Creates DelegatedRole object from its dict representation."""
         name = role_dict.pop("name")
         keyids = role_dict.pop("keyids")