PY-57541 PY-53896 Add missing AnyStr_co to partially updated Typeshed

Its absence made Py3TypeCheckerInspectionTest.testPathlibPathMatchingOsPathLike
fail as os.PathLike protocol wasn't considered generic. Once the changes
for generics are in the upstream, we will be able to update the Typeshed
fully at long last.

GitOrigin-RevId: 1333cc19bff15e328996c57362124a7fdb5ab057

python/helpers/typeshed/stdlib/_typeshed/__init__.pyi

@@ -23,6 +23,9 @@ _T_contra = TypeVar("_T_contra", contravariant=True)
 #   def __enter__(self: Self) -> Self: ...
 Self = TypeVar("Self")  # noqa Y001
 
+# covariant version of typing.AnyStr, useful for protocols
+AnyStr_co = TypeVar("AnyStr_co", str, bytes, covariant=True)  # noqa: Y001
+
 # stable
 class IdentityFunction(Protocol):
     def __call__(self, __x: _T) -> _T: ...