8294: remove sorting from polyroots
avoid using roots(multiple=True) if possible

- add test for DiracDelta simplification involving several roots
  and simplify tests in its computation
8285,8316: keep off the y axis during isolation

Although there are NotImplemented aspects to the algorithm,
the fix here avoids some of those corners.

An arbitrary 1/3:2/3 split is used instead of 1/2:1/2 if
splitting an interval leads to an x value of 0. There should
now never be a 0 in the x coordinates of interval rectangles.
use I and pi instead of S.I and S.pi
8306: make convergence error message friendlier
modify docstring [skip ci]
Merge pull request #8337 from smichr/8294

8294: keep sorting of roots but remove sorting in private methods of polyroots