Multiply Python pair by Rational multiplier. Hold pair numerator constant, if possible. Return new Python pair representing product.
Intended for certain types of meter multiplication.
Examples:
durtools.pair_multiply_constant_numerator(9, 16), Rational(2, 3)) (9, 24)
durtools.pair_multiply_constant_numerator(9, 16), Rational(1, 2)) (9, 32)
durtools.pair_multiply_constant_numerator(9, 16), Rational(5, 6)) (45, 96)