input: Write a function to evaluate simple mathematical expressions given as strings.
  assert evaluate_expression("3 + 4") == 7
  assert evaluate_expression("10 - 5") == 5
  assert evaluate_expression("6 * 2") == 12
  assert evaluate_expression("8 / 2") == 4.0
output: def evaluate_expression(expression: str) -> float:
      return eval(expression)