choice(range(start, stop, step)) but faster.  This addresses the
problem that randint() was accidentally defined as taking an inclusive
range (how unpythonic).

The code is longish because Tim Peters insisted that it reject
non-integral arguments while I insisted that it be not much slower
than randint(); the compromise satisfies both but is somewhat
convoluted.

Also changed randint() to be implemented through randrange().  This is
a semantic change because old randint() didn't test its arguments for
validity.  (It also makes randrange() win any contest with randint()
:-)