sage.structure.coerce.py_scalar_parent, py_scalar_to_element: Make dynamic

[mkoeppe]

We make it possible to register a different numerical tower instead, such as a tower built on mock integers (#29874).

This needs to be benchmarked to make sure that there are no performance regressions.

Component: refactoring

Issue created by migration from https://trac.sagemath.org/ticket/29875

[mkoeppe]

comment:2

Setting new milestone based on a cursory review of ticket status, priority, and last modification date.