bpo-39763: Add _bootsubprocess to build Python on AIX (GH-18872)

Add _bootsubprocess module to bootstrap Python: subprocess
implementation which only uses the os module.

On AIX, distutils.util uses _aix_support which calls
subprocess.check_output(), before the _posixsubprocess module is
built. Implement check_output() with os.system() in _bootsubprocess.
2 files changed