Metadata API: Fix verify_delegate() annotation

mypy rules are stricter now.

Signed-off-by: Jussi Kukkonen <[email protected]>

Author: Jussi Kukkonen

tuf/api/metadata.py

@@ -274,7 +274,7 @@ def verify_delegate(
         delegated_role: str,
         delegated_metadata: "Metadata",
         signed_serializer: Optional[SignedSerializer] = None,
-    ):
+    ) -> None:
         """Verifies that 'delegate' is signed with the required threshold of
         keys for the delegated role 'role_name'.