bpo-32968: Make modulo and floor division involving Fraction and float consistent with other operations (#5956)

Make mixed-type `%` and `//` operations involving `Fraction` and `float` objects behave like all other mixed-type arithmetic operations: first the `Fraction` object is converted to a `float`, then the `float` operation is performed as normal. This fixes some surprising corner cases, like `Fraction('1/3') % inf` giving a NaN.

Thanks Elias Zamaria for the patch.