bpo-41056: Use the fildes converter for fd to please Coverity. (GH-21011)

There are a bunch of other fd: int uses in this file, I expect many if not
all of them would be better off using the fildes converter.  This particular
one was flagged by Coverity as it presumably flags fpathconf as not accepting
negative fds.  I'd expect the other fd's to have been flagged as well
otherwise.

I'm marking this one as skip news as it really is a no-op.