posix_spawn can still be found in Python 3.7:
configure:11243: posix_fallocate posix_fadvise posix_spawn pread preadv preadv2 \
configure.ac:3470: posix_fallocate posix_fadvise posix_spawn pread preadv preadv2 \
pyconfig.h.in:710:/* Define to 1 if you have the `posix_spawn' function. */
This check has been added by the commit 6c6ddf97c402709713d668d0ed53836a7749ba99 and should be removed from Python 3.7 as well, no? It's not a release blocker, but it may be good to remove posix_spawn from configure.ac (and related generated files).