input: Write a function to calculate the sum of user-entered integers until 0 is entered.
  assert calculate_sum([1, 2, 3, 0]) == 6
  assert calculate_sum([10, -5, 0]) == 5
  assert calculate_sum([0]) == 0
output: def calculate_sum(numbers):
      total = 0
      for num in numbers:
          if num == 0:
              break
          total += num
      return total