> I don't understand why any patch for CPython is needed at all.

If and operation on self._handle fails as an invalid handle (due to an underlying STATUS_INVALID_HANDLE or STATUS_OBJECT_TYPE_MISMATCH), then call self._handle.Detach() and re-raise the exception.

It wouldn't hurt to also address the case in which coincidentally the handle value currently references another process. When the process is spawned, save the (process_id, creation_time) via GetProcessId() and GetProcessTimes(). Implement a function that validates these values before calling WaitForSingleObject(), GetExitCodeProcess(), or TerminateProcess(). If validation fails, call self._handle.Detach() and raise OSError.