Tests for the Poly.roots property
Function lazy_math.sign on Python 3 + tests
OrderedDict-based Poly, order kept on operations
Cached frozenset-based Poly hash (incl. Poly.zero)
Poly ordered on unary op; xzip_longest on tests
Poly.terms sort and reverse keyword arguments
Horner scheme now optional (new Poly call kwarg)
Testing complex & Sympy symbols as Poly powers
Fix Poly __setitem__ to zero & __init__ w/ zero!=0
Poly zero assignment cleaning & str representation