bpo-37116: Use PEP 570 syntax for positional-only parameters. (GH-13700)

diff --git a/Lib/typing.py b/Lib/typing.py
index 16ccfad..5f1a0ad 100644
--- a/Lib/typing.py
+++ b/Lib/typing.py
@@ -283,7 +283,7 @@
 
     __slots__ = ('__weakref__',)
 
-    def __init_subclass__(self, *args, **kwds):
+    def __init_subclass__(self, /, *args, **kwds):
         if '_root' not in kwds:
             raise TypeError("Cannot subclass special typing classes")