bpo-20104: Expose `posix_spawn` in the os module (GH-5109)

Add os.posix_spawn to wrap the low level POSIX API of the same name.

Contributed by Pablo Galindo.
diff --git a/configure b/configure
index 6ce0728..f94d16b 100755
--- a/configure
+++ b/configure
@@ -11197,7 +11197,7 @@
  initgroups kill killpg lchmod lchown lockf linkat lstat lutimes mmap \
  memrchr mbrtowc mkdirat mkfifo \
  mkfifoat mknod mknodat mktime mremap nice openat pathconf pause pipe2 plock poll \
- posix_fallocate posix_fadvise pread preadv preadv2 \
+ posix_fallocate posix_fadvise posix_spawn pread preadv preadv2 \
  pthread_init pthread_kill putenv pwrite pwritev pwritev2 readlink readlinkat readv realpath renameat \
  sem_open sem_timedwait sem_getvalue sem_unlink sendfile setegid seteuid \
  setgid sethostname \