Fix stub: Allow "Exists" key in assert_json_subset

Author: srittau

asserts/__init__.pyi

@@ -159,7 +159,7 @@ def assert_warns_regex(
     warning_type: type[Warning], regex: str, msg_fmt: str = ...
 ) -> AssertWarnsContext: ...
 def assert_json_subset(
-    first: dict[str, Any] | list[Any],
+    first: dict[Any, Any] | list[Any],  # dict key can be 'str' or 'Exists'
     second: dict[str, Any] | list[Any] | str | bytes,
 ) -> None: ...