input: Write a function to evaluate and return the result of a numerical expression given as a string.
  def eval_expr(expr):
     return eval(expr)
  
  assert eval_expr("10 + 20 * 3") == 70
  assert eval_expr("(10 + 20) * 3") == 90
  assert eval_expr("4 ** 2") == 16
  assert eval_expr("7 / 3.0") == 2.3333333333333335
  assert eval_expr("0") == 0
output: def eval_expr(expr):
     return eval(expr)