[3.9] gh-122133: Authenticate socket connection for `socket.socketpair()` fallback...

[3.9] gh-122133: Authenticate socket connection for `socket.socketpair()` fallback (GH-122134) (#122428)

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).

(cherry picked from commit 78df1043dbdce5c989600616f9f87b4ee72944e5)

Co-authored-by: Seth Michael Larson <seth@python.org>
Co-authored-by: Gregory P. Smith <greg@krypto.org>