bpo-35491: Enhance multiprocessing.BaseProcess.__repr__() (GH-11138)

* Add the pid and parent pid to multiprocessing.BaseProcess.__repr__().
* Add negative sign (ex: "-SIGTERM") to exitcode (process killed
  by a signal)
* Only call _popen.poll() once.

Example:
  <ForkProcess(ForkPoolWorker-1, started daemon)>
becomes:
  <ForkProcess name='ForkPoolWorker-1' pid=12449 parent=12448 started daemon>

Example:
  <ForkProcess(ForkPoolWorker-1, stopped[SIGTERM] daemon)>
becomes:
  <ForkProcess name='ForkPoolWorker-1' pid=12960 parent=12959 stopped exitcode=-SIGTERM daemon>