bpo-39481: Fix duplicate SimpleQueue type in test_genericalias.py (GH-22619)

There are two different `SimpleQueue` types imported (from `multiprocessing.queues` and `queue`) in `Lib/test/test_genericalias.py`, the second one shadowing the first one, making the first one not actually tested. Fix by using different names.

Automerge-Triggered-By: @gvanrossum