Add abstract _add_ and _mul_ methods to RingElement

[jdemeyer]

In order to fix all Compatible but non-identical C method '_add_' not redeclared in definition part of extension type '...'. This may cause incorrect vtables to be generated. warnings, we add abstract methods _add_ and _mul_ to some base Element classes.

This is part of #23600.

In #24116, such abstract methods were added to the finite ring element base class. Those can be removed again.

CC: @tscrim

Component: cython

Author: Jeroen Demeyer

Branch/Commit: d984df5

Reviewer: Travis Scrimshaw

Issue created by migration from https://trac.sagemath.org/ticket/24607

[jdemeyer]

Description changed:

--- 
+++ 
@@ -1 +1,3 @@
+In order to fix all `Compatible but non-identical C method '_add_' not redeclared in definition part of extension type '...'.  This may cause incorrect vtables to be generated.` warnings, we add abstract methods `_add_` and `_mul_` to some base `Element` classes.
 
+See #23600 and #24116.

[jdemeyer]

Description changed:

--- 
+++ 
@@ -1,3 +1,5 @@
 In order to fix all `Compatible but non-identical C method '_add_' not redeclared in definition part of extension type '...'.  This may cause incorrect vtables to be generated.` warnings, we add abstract methods `_add_` and `_mul_` to some base `Element` classes.
 
-See #23600 and #24116.
+This is part of #23600.
+
+In #24116, such abstract methods were added to the finite ring element base class. Those can be removed again.

[jdemeyer]

Branch: u/jdemeyer/add_abstract__add__and__mul__methods_to_ringelement

[jdemeyer]

Commit: 8180e22

[jdemeyer]

New commits:

8180e22

Add abstract `_add_` and `_mul_` methods

[tscrim]

comment:5

I know it doesn't really matter, but the error string for the NotImplementedError I think is not formatted properly (missing the leading f).

[tscrim]

comment:6

Otherwise LGTM and you can set a positive review on my behalf either way.

[tscrim]

Reviewer: Travis Scrimshaw

[jdemeyer]

comment:7

Ooops...

[sagetrac-git]

Branch pushed to git repo; I updated commit sha1. This was a forced push. New commits:

4715809

Add abstract `_add_` and `_mul_` methods