Make polynomials respect the _gcd framework so that they can use coercion in gcds

[robertwb]

In support of #7585

CC: @roed314

Component: algebra

Author: David Roe

Reviewer: Robert Bradshaw

Issue created by migration from https://trac.sagemath.org/ticket/7881

[robertwb]

comment:1

Attachment: 7585_6_gcd.patch.gz

I'm all for using the coercion model more, but I don't like the fact that this hides the important docstrings about ZZ[x] and RR[x] in the underscore methods.

[robertwb]

comment:2

I think this is taken care of with #383.

[loefflerd]

comment:3

I propose closing this as "duplicate".

[robertwb]

comment:4

I concur.