Okay, I got inspired and (in the words of Barry Warsaw) JFDI. Attached is my revised patch. I took Serhiy's patch and reworked it quite a bit:
* I think it's now easier to follow. In particular:
* The most common case (no overflow) is now first. In Serhiy's patch
the most common case is buried in the middle of the second "if".
* I removed some extraneous tests.
* I changed the error messages to call the values "uid" and "gid", to match the names of the parameters.
* I noticed that _fd_converter had the same bad-idea PyFloat_Check, so I changed it to use PyNumber_Index instead.
In the process I also noticed that Serhiy's approach had a resource leak: it never decref'd the result of PyNumber_Index() when successful.
To make sure I duplicated Serhiy's semantics, I had a test harness that ran both his and mine and ensured they returned the same thing (or both threw an error). Obviously I removed all that before cutting the patch.