[3.13] gh-89364: Export PySignal_SetWakeupFd() function (GH-121537) (#121582)

gh-89364: Export PySignal_SetWakeupFd() function (GH-121537)

Export the PySignal_SetWakeupFd() function. Previously, the function
was documented but it couldn't be used in 3rd party code.
(cherry picked from commit ca0fb3423c13822d909d75eb616ecf1965e619ae)

Co-authored-by: Victor Stinner <vstinner@python.org>