py3: suppress unraisable exceptions in test-worker.t Python 3.8 calls sys.unraisablehook when an unraisable exception is encountered. The default behavior is to print a warning. test-worker.t was triggering this hook due to a race between a newly forked process exiting and that process's _os.register_at_fork handlers running. I was seeing the stdlib's random module in the stack re-seeding itself. Although there could be other after-fork handlers in the mix. This commit defines sys.unraisablehook to effectively no-op. This suppresses the warning and makes test output on Python 3.8 consistent with prior versions. test-worker.t now passes on Python 3.8. Differential Revision: https://phab.mercurial-scm.org/D7949