Add Digits Python Solution with Tests

lcpy gen -n 258   # by problem number
lcpy gen -s add_digits   # by problem name

Input: num = 38
Output: 2
Explanation: The process is
38 --> 3 + 8 --> 11
11 --> 1 + 1 --> 2
Since 2 has only one digit, return it.

Input: num = 0
Output: 0

class Solution:
    # Time: O(1) (at most 3 passes for 32-bit inputs)
    # Space: O(1)
    def add_digits(self, num: int) -> int:
        if num == 0:
            return 0
        return 1 + (num - 1) % 9