Add is_algebraic assumption

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

algebraic + transcendental is transcendental. Algebraic numbers are closed under addition and multiplication (algebraic + algebraic = algebraic and algebraic*algebraic = algebraic), so if algebraic + transcendental = algebraic, then by rearranging the equation you would get algebraic - algebraic = transcendental, which is not the case.  It's a similar line of reasoning that shows that `1 + sqrt(2)` is irrational once you know that `sqrt(2)` is irrational. In terms of SymPy it's sort of an "inverted closed group" idea. 

Transcendental + transcendental is not transcendental in general, because things could cancel (like `pi + (-pi)`). Even in cases where things don't appear to cancel like `E + pi` it is often not known, because it isn't proven that there isn't some "deep" cancellation going on (in particular, you have to show that *any* polynomial with rational coefficients of `E + pi` doesn't cancel to zero; `pi + (-pi)` is a linear polynomial).