정적분 $\displaystyle \int _0^ {\frac{\pi} {3}} {\sec ^ {2021} x \tan x dx}$ 의 값은?

• ①$2^ {2020} -1$
• ②$2^ {2020} +1$
• ③$\displaystyle \frac{2^ {2021} -1} {2021}$
• ④$\displaystyle \frac{2^ {2021} +1} {2021}$