Answer
The integral does not exist.
Work Step by Step
$\int_{\pi/3}^{\pi} sec~\theta~tan~\theta~d\theta = \int_{\pi/3}^{\pi} \frac{sin~\theta}{cos^2~\theta}~d\theta$
The graph of $~~\frac{sin~\theta}{cos^2~\theta}~~$ has a vertical asymptote at the point $\theta=\frac{\pi}{2}$. Therefore, the function is not continuous on the interval $[\frac{\pi}{3},\pi]$ and we can not apply the Fundamental Theorem to calculate the value of this integral.
The integral does not exist.