[3.13] gh-136006: fix `Py_NAN` expansion on Solaris systems (GH-136575) (#138734)

On Solaris, `Py_NAN` may expand as a function address instead of a floating-point number.
This amends commit 7a3b03509e5e3e72d8c47137579cccb52548a318.
(cherry picked from commit d54b1091d43b9d8f0da0ba081565bccca3f138fd)