input: Write a python function to calculate the sum of digits of a number.
  assert sum_of_digits(123) == 6
  assert sum_of_digits(4567) == 22
  assert sum_of_digits(0) == 0
  assert sum_of_digits(987654321) == 45
output: def sum_of_digits(number):
      return sum(int(digit) for digit in str(number))