For the modularization, it's better to do the numbers ABC registration in the individual implementation modules.

We keep the module sage.rings.numbers_abc just for the doctests.

CC: @tscrim

Component: refactoring

Author: Matthias Koeppe

Branch/Commit: 8024209

Reviewer: Travis Scrimshaw

Issue created by migration from https://trac.sagemath.org/ticket/32641