Yes, the problem is that the pipes FDs are closed twice if fork() succeeds, but exec() fails.

They're closed in _execute_child():
  1286                 if p2cread is not None and p2cwrite is not None:
  1287                     os.close(p2cread)
  1288                 if c2pwrite is not None and c2pread is not None:
  1289                     os.close(c2pwrite)
  1290                 if errwrite is not None and errread is not None:
  1291                     os.close(errwrite)

And then again in Popen.__init__:
   716             if stdin == PIPE:
   717                 to_close.extend((p2cread, p2cwrite))
   718             if stdout == PIPE:
   719                 to_close.extend((c2pread, c2pwrite))
   720             if stderr == PIPE:
   721                 to_close.extend((errread, errwrite))