pynac: Replace direct use of singular's libfactory by a Python call

[mkoeppe]

pynac has a dependency on libfactory's CanonicalForm class, for polynomial factorization, expansion, gcd, resultant.

We should replace this by a callback to a Python function.

$ git grep factory/factory src/sage/symbolic/
src/sage/symbolic/expression.pyx:#   factory/factory.h    so this ^ is needed to find it
src/sage/symbolic/ginac/mpoly-singular.cpp:#include "factory/factory.h"
src/sage/symbolic/ginac/numeric.cpp:#include "factory/factory.h"

Component: symbolics

Issue created by migration from https://trac.sagemath.org/ticket/32613