> allows Fraction instantiation from duck-typing classes that provide as_integer_ratio

This would allow the numpy `np.floating` types to take part in `Fraction` conversion as well, which would be great.

As far as I can tell, `Decimal` already follows this duck-typing, so it would be a shame for other modules not to.

Perhaps an `num, den = operator.rational(x)` is needed to unify this code across the places that coerce rational numbers.