Write a function that will create and return a Python list of the prime factorization of a positive integer n.
That is, factor(36) should return the list [2, 2, 3, 3] and factor(7) should return the single-element list [7].
factor(1) should return [1] but otherwise there should never be a 1 in the returned list.
Hint: there's a way to do this without needing to test if any numbers are prime.
©2017 Graham Mitchell