[3.9] bpo-37116: Use PEP 570 syntax for positional-only parameters. (GH-12620)

Turn deprecation warnings added in 3.8 into TypeError.
diff --git a/Doc/library/profile.rst b/Doc/library/profile.rst
index d8039fd..8d589d2 100644
--- a/Doc/library/profile.rst
+++ b/Doc/library/profile.rst
@@ -309,7 +309,7 @@
       Profile the cmd via :func:`exec` with the specified global and
       local environment.
 
-   .. method:: runcall(func, *args, **kwargs)
+   .. method:: runcall(func, /, *args, **kwargs)
 
       Profile ``func(*args, **kwargs)``