bpo-33097: Fix submit accepting callable after executor shutdown by interpreter exit...

bpo-33097: Fix submit accepting callable after executor shutdown by interpreter exit (GH-6144) (GH-6445)

Executors in concurrent.futures accepted tasks after executor was shutdown by interpreter exit. Tasks were left in PENDING state forever. This fix changes submit to instead raise a RuntimeError.
(cherry picked from commit c4b695f85e141f57d22d8edf7bc2c756da136918)

Co-authored-by: Mark Nemec <mrknmc@me.com>