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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