ignore another mypy finding

Signed-off-by: William Woodruff <william@trailofbits.com>

Author: woodruffw

sigstore/models.py

@@ -578,7 +578,7 @@ def _dsse_envelope(self) -> dsse.Envelope | None:
         @private
         """
         if self._inner.is_set("dsse_envelope"):
-            return dsse.Envelope(self._inner.dsse_envelope)
+            return dsse.Envelope(self._inner.dsse_envelope)  # type: ignore[arg-type]
         return None
 
     @property