As a big first step towards #14329, I propose to remove all imports of methods is_[A-Z] in sage/rings/all.py

CC: @ppurka @nathanncohen

Component: build

Keywords: deprecation cleanup

Author: Frédéric Chapoton

Reviewer: Nathann Cohen

Merged: sage-5.13.beta1

Issue created by migration from https://trac.sagemath.org/ticket/15076