Add _complement helper for EmptySet, rename few sets methods from complement to _complement
Fix solve_poly_inequality for constant polys

Was:
In [5]: solve_poly_inequality(Poly(0, x), '==')
Out[5]: []

In [6]: solve_poly_inequality(Poly(1, x), '==')
Out[6]: []
reduce_rational_inequalities: substitute correct polys for True/False

fixes #8545
Cleanup solve_rational_inequalities
Cleanup: remove extra imports from doctests of inequalities.py
Merge pull request #8550 from skirpichev/8545-fix

reduce_rational_inequalities: substitute correct polys for True/False