Attached is a patch based on Chris Barker's implementation on github[1]. This includes only the C implementation, as well as tests, documentation and entries in NEWS and whatsnew.

I had to keep the (PyCFunction) cast in the module method list in Modules/mathmodule.c. That part should certainly be reviewed.

As for documentation etc., I took the best wording I could find from the PEP and the docs in the github repo, and made very slight changes only where I though they made things significantly clearer.

.. [1]: https://github.com/PythonCHB/close_pep