gh-122133: Authenticate socket connection for `socket.socketpair()` fallback (GH-122134)

* Authenticate socket connection for `socket.socketpair()` fallback when the platform does not have a native `socketpair` C API.  We authenticate in-process using `getsocketname` and `getpeername` (thanks to Nathaniel J Smith for that suggestion).

Co-authored-by: Gregory P. Smith <greg@krypto.org>