[3.14] gh-143698: correctly check `scheduler` and `setpgroup` values for `os.posix_sp...

[3.14] gh-143698: correctly check `scheduler` and `setpgroup` values for `os.posix_spawn[p]` (GH-143699) (#145073)

Fix an issue where passing invalid arguments to `os.posix_spawn[p]` functions
raised a SystemError instead of a TypeError, and allow to explicitly use `None`
for `scheduler` and `setpgroup` as specified in the docs.

(cherry picked from commit 347fc438cf903c1d7fa5063464ae2e93c11b2232)