Merge pull request #1450 from MVrachev/threshold-validation

Metadata API: Add simple threshold validation

tuf/api/metadata.py

@@ -522,6 +522,8 @@ def __init__(
                 f"keyids should be a list of unique strings,"
                 f" instead got {keyids}"
             )
+        if threshold < 1:
+            raise ValueError("threshold should be at least 1!")
         self.keyids = keyids_set
         self.threshold = threshold
         self.unrecognized_fields: Mapping[str, Any] = unrecognized_fields or {}