__floordiv__ for sparse polynomials #34653
Description
Before this ticket, the following would raise an ArithmeticError in the sparse case:
sage: P.<x,y> = PolynomialRing(SR)
sage: R.<z> = PolynomialRing(P, sparse=False); R
Univariate Polynomial Ring in z over Multivariate Polynomial Ring in x, y over Symbolic Ring
sage: (x*z) // x
z
sage: R.<z> = PolynomialRing(P, sparse=True); R
Sparse Univariate Polynomial Ring in z over Multivariate Polynomial Ring in x, y over Symbolic Ring
sage: (x*z) // x
z
Component: algebra
Author: Martin Rubey
Branch/Commit: 9577386
Reviewer: Travis Scrimshaw
Issue created by migration from https://trac.sagemath.org/ticket/34653