Issue #13812: When a multiprocessing Process child raises an exception, flush stderr after printing the exception traceback.

Issue #13812: When a multiprocessing Process child raises an exception, flush stderr after printing the exception traceback.

This required updating the code to use posix instead of os. This is
all being done to make bootstrapping easier to removing dependencies
that are kept in importlib.__init__ and thus outside of the single
file to bootstrap from.

Skip the test if we are unable to find an invalid time_t value.

is not available or its value cannot be represented

of ValueError on failure. time.ctime() and time.asctime() now raises an
OSError if localtime() failed.

does not need to be exposed from C code for bootstrapping reasons.

Closes #13859: Replaced reference to StandardError with reference to Exception. Thanks to Matt Joiner for spotting this and submitting a patch.

(rather than `-m test.regrtest`)