Add stubs for posix_spawn and posix_spawnp (#5846)

stdlib/os/__init__.pyi

@@ -819,6 +819,8 @@ else:
     def WEXITSTATUS(status: int) -> int: ...
     def WSTOPSIG(status: int) -> int: ...
     def WTERMSIG(status: int) -> int: ...
+    if sys.version_info >= (3, 8):
+        from posix import posix_spawn as posix_spawn, posix_spawnp as posix_spawnp
 
 if sys.platform != "win32":
     from posix import sched_param