[2.7] bpo-28994: Fixed errors handling in atexit._run_exitfuncs(). (GH-2034) (#2123)

The traceback no longer displayed for SystemExit raised in a callback registered by atexit..
(cherry picked from commit 3fd54d4a7e604067e2bc0f8cfd58bdbdc09fa7f4)
3 files changed