The use of large literal numbers is a big no-no when it comes to writing
programs which work with different int representations. Also, some checks
are pretty adhoc (e.g using struct module to check for 64-bitness). This
change bases entire detection on sys.maxsize and integer operarions, and
thus more correct, even if longer.
Note that this change doesn't mean that any of these tests can pass with
anything but MPZ - even despite checking for various int representations,
the tests aren't written to be portable among them.
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