input: Write a function to evaluate a numerical expression given as a string and return the result of the expression.
  assert eval_expr('1 + 2') == 3
  assert eval_expr('5 * 3') == 15
  assert eval_expr('8 / 2') == 4
  assert eval_expr('2 ** 3') == 8
  assert eval_expr('10 - 4') == 6
output: def eval_expr(expression):
      return eval(expression)