input: Write a python function to sum all the numbers in a given expression string.
  assert sum_expression("2 + 4 - 7 + 3 + 4 - 8") == 8
  assert sum_expression("1 + 1 + 1 + 1 + 1") == 5
  assert sum_expression("10 + 20 + 30") == 60
output: def sum_expression(expr):
      return sum(int(num) for num in expr.split() if num not in '+-')