subprocess triggers a debug assertion in the CRT on failure to execute the process due to closing the pipe *handles* in the except clause using os.close rather than .Close() (os.close closes CRT file descriptors and not handles).

In addition to that once this is fixed there is also a double free/close since we need to set `self._closed_child_pipe_fds = True` once we closed the handles in _execute_child so they won't also be closed in __init__.

To reproduce, do this in a debug build of Python:

    import subprocess
    subprocess.Popen('exe_that_doesnt_exist.exe', stdout=subprocess.PIPE)

See: https://github.com/python/cpython/pull/1218#discussion_r112550959