Given
an unsorted list, create a function that returns the nth smallest
integer (the smallest integer is the first smallest, the second
smallest integer is the second smallest, etc).
Examples
nth_smallest([1, 3, 5, 7], 1) ➞ 1
nth_smallest([1, 3, 5, 7], 3) ➞ 5
nth_smallest([1, 3, 5, 7], 5) ➞ None
nth_smallest([7, 3, 5, 1], 2) ➞ 3