Sum of divisors

Write a function `sum_of_divisors(n: int) -> int` that returns the sum of all positive divisors of the integer `n`. For example, the divisors of 6 are 1, 2, 3, and 6, so the sum is 12. Your solution should handle `n` up to 10^6 efficiently; a simple loop up to `n` is acceptable but try to optimize by iterating only up to the square root.

>>> sum_of_divisors(1)
1
>>> sum_of_divisors(6)
12
>>> sum_of_divisors(12)
28
>>> sum_of_divisors(0)
0