input: Write a function that takes a string containing a numerical expression and returns the result.
  
  assert eval_expr("2 + 3 * 4") == 14
  assert eval_expr("5 - 2") == 3
  assert eval_expr("6 / 3") == 2
  assert eval_expr("2 ** 3") == 8
  assert eval_expr("20 % 7") == 6
output: def eval_expr(expression):
      return eval(expression)