verify: Add temporary mypy ignore

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

Author: jku

sigstore/verify/verifier.py

@@ -140,7 +140,8 @@ def _verify_signed_timestamp(
 
             verifier = builder.build()
             try:
-                verifier.verify_message(timestamp_response, message)
+                # TODO: remove ignore after rfc3161-client upgrade
+                verifier.verify_message(timestamp_response, message)  # type: ignore[attr-defined]
             except Rfc3161VerificationError as e:
                 _logger.debug("Unable to verify Timestamp with CA.")
                 _logger.exception(e)