polys: Removed duplicated code from GroebnerBasis.__new__ (#3686)
polys: DRY in sympy/polys/domains/__init__.py
utilities: Added polys' __init__s to implicit imports test's ignore list
polys: More useful constructors of PolynomialRing and FractionField
polys: Made *poly_from_expr() independent of DMP
polys: Added dummy sfield() function to make API symmetric
polys: Use __nonzero__ instead of __bool__ and let 2to3 do its job
polys: Don't use __slots__ in GroebnerBasis