Sum of Digits
The task in this mission is as follows:
You are given an integer. If it consists of one digit, simply return its value. If it consists of two or more digits - add them until the number contains only one digit and return it.
Input: Integer (int).
Output: Integer (int).
Examples:
assert sum_digits(38) == 2 assert sum_digits(0) == 0 assert sum_digits(10) == 1 assert sum_digits(132) == 6
