Fix small pylint error

Signed-off-by: Martin Vrachev <[email protected]>

Author: MVrachev

tuf/api/metadata.py

@@ -539,7 +539,7 @@ def __init__(
     ):
         if not all(
             isinstance(at, str) for at in [keyid, keytype, scheme]
-        ) or not isinstance(keyval, Dict):
+        ) or not isinstance(keyval, dict):
             raise TypeError("Unexpected Key attributes types!")
         self.keyid = keyid
         self.keytype = keytype