[jsonschema] Mark RefResolver as deprecated (#15546)

Author: srittau

stubs/jsonschema/jsonschema/validators.pyi

@@ -96,6 +96,14 @@ class Draft202012Validator(_Validator):
 
 _Handler: TypeAlias = Callable[[str], Incomplete]
 
+@deprecated(
+    "jsonschema.RefResolver is deprecated as of v4.18.0, in favor of the "
+    "https://github.com/python-jsonschema/referencing library, which "
+    "provides more compliant referencing behavior as well as more "
+    "flexible APIs for customization. A future release will remove "
+    "RefResolver. Please file a feature request (on referencing) if you "
+    "are missing an API for the kind of customization you need."
+)
 class RefResolver:
     referrer: dict[str, Incomplete]
     cache_remote: Incomplete