Very simple fix for issue #8235

In solve_poly_inequality, the real roots of a polynomial are sorted. When the
roots are instances of RootOf, this will invoke bool() on an instance of
Relational, which raises a TypeError. One fix is to cast all the roots as
floats for comparison.

Fixes #8235
>> Why do you think it's not the case?
Why do I think it's the case that a function doesn't necessarily imply an order? Real numbers have a natural order, but in many cases the roots returned from these functions aren't necessarily real -- as far as I can tell they could be any combination of real, complex, or symbolic. Unless sympy has an existing convention for ordering complex numbers and symbolic expressions, we shouldn't impose one outside of `default_sort_key`. Any other ordering will be equally as arbitrary and perhaps more unexpected.

The caveat here is that in the case where these functions return only real numbers, perhaps the natural order for reals should be used.