Improve _eval_is_algebraic helper for trigonometric numbers

Now sin(pi*rational) or cos(pi*rational) - is_algebraic is True,
see: http://mathworld.wolfram.com/TrigonometryAngles.html

fixes #8349
Merge pull request #8351 from skirpichev/improve-trig-is_algebraic

Improve _eval_is_algebraic helper for trigonometric numbers