jsonschema: fix type of validator on errors

I noticed this while improving mypy's reachability analysis in python/mypy#20660 (because the type of validator is wrong, mypy would think some code is unreachable)

Author: hauntsaninja

stubs/jsonschema/jsonschema/exceptions.pyi

@@ -21,7 +21,7 @@ class _Error(Exception):
     relative_schema_path: deque[str | int]
     context: list[ValidationError]
     cause: Exception | None
-    validator: Validator | Unset
+    validator: str | Unset
     validator_value: Any | Unset
     instance: Any | Unset
     schema: Mapping[str, Any] | bool | Unset