bpo-36170: posix_spawn doesn't exist on 3.7 (GH-12143)

The 3.8 docs claim that `os.posix_spawn` was introduced in 3.7, but it wasn't; it will be introduced in 3.8.

Doc/library/os.rst

@@ -3465,7 +3465,7 @@ written in Python, such as a mail server's external command delivery program.
    :c:data:`POSIX_SPAWN_SETSCHEDPARAM` and :c:data:`POSIX_SPAWN_SETSCHEDULER`
    flags.
 
-   .. versionadded:: 3.7
+   .. versionadded:: 3.8
 
    .. availability:: Unix.