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
shorten lines and correct test
inequalities: minor edits
edit inequalities error messages
don't rewrite with re and im parts

By rewriting in terms of re and im parts allows the resulting expression
to give False instead of the proper error that should occur if something
with im(x) != 0 is used in a Relational.
reduce_rational_inequalities
discard (-oo, oo) from Intersection

Without this commit, `bin/test contin -k _P` will fail in the
previous commit. After the next commit that changes
solve_univariate_inequality, the test will no longer fail but
the issue still exists: (-oo, oo) is not removed.
solve_univariate_inequality
update tests
edit error message
recast vanilla symbols as reals
quick