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
My 2 cents: unless a function implies or has explicit options to return a different ordering, use `default_sort_key` to return the canonical ordering. Seems a lot of existing code already expects this behavior. If you require a certain non-canonical ordering, you should order the roots yourself. Another solution could be to add options to the "offending" roots functions to have them return different orders?