Clamp a quantity

Orders must be for at least 1 and at most 20 of an item. Write clamp_quantity(q) that returns q unchanged if it is already in range, 1 if it is below, and 20 if it is above.

clamp_quantity(7)   ->  7
clamp_quantity(0)   ->  1
clamp_quantity(99)  ->  20

def clamp_quantity(q):
    ...