This new module will provide abstract base classes for the purpose of isinstance and issubclass testing. This helps with modularization (#32432).

In this ticket, we add RealField, RealDoubleField, ComplexField, ComplexDoubleField, which are in 1:1 correspondence to the implementation classes RealField_class, RealDoubleField_class, ComplexField_class, ComplexDoubleField_class.

To illustrate the use of these classes, we convert some uses of is_... functions to isinstance with the new base classes.

Related:

• https://groups.google.com/g/sage-devel/c/T0A4JCOg9DY/m/q6WKsI1yBAAJ (Sep 2021)

Part of meta-ticket #32414.

CC: @tscrim @kliem @videlec @nbruin

Component: refactoring

Author: Matthias Koeppe

Branch/Commit: 6e2c3c4

Reviewer: Jonathan Kliem

Issue created by migration from https://trac.sagemath.org/ticket/32566