gh-119594: Improve pow(fraction.Fraction(), b, modulo) error message (#119593)

If one calls pow(fractions.Fraction, x, module) with modulo not None, the error message now says that the types are incompatible rather than saying pow only takes 2 arguments.  Implemented by having fractions.Fraction __pow__ accept optional modulo argument and return NotImplemented if not None.  pow() then raises with appropriate message.
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Co-authored-by: Mark Dickinson <dickinsm@gmail.com>