Add is_algebraic assumption

for some other changes in sympy.polys - see #7898

> exp(q*pi) is transcendental for any q rational different from zero

Indeed, good point.

But, @pernici, I'm about my other changes in the polys module (commit 57aac6a).  In the current codebase - we sometimes use new assumptions (and old too!), I would like to remove that.  Can you review this change?

In the current code