Use coercion instead of pushout in matrix actions

[videlec]

There is no coercion between QQ and finite rings

sage: cm = get_coercion_model()
sage: cm.get_action(Zmod(2), QQ, operator.mul) is None
True

This is right.

But we do have an action of matrices on vectors

sage: M = MatrixSpace(Zmod(2),2)
sage: V = VectorSpace(QQ,2)
sage: cm.get_action(M,V,operator.mul)
Left action by
  Full MatrixSpace of 2 by 2 dense matrices over Ring of integers modulo 2 on
  Vector space of dimension 2 over Rational Field

or

sage: M = MatrixSpace(QQ,2)
sage: V = VectorSpace(Zmod(2),2)
sage: cm.get_action(M,V,operator.mul)
Left action by
  Full MatrixSpace of 2 by 2 dense matrices over Rational Field on
  Vector space of dimension 2 over Ring of integers modulo 2

And this is wrong!

see also: #12865

Component: coercion

Author: Jeroen Demeyer

Branch/Commit: 9da3db6

Reviewer: Travis Scrimshaw

Issue created by migration from https://trac.sagemath.org/ticket/17859

[jdemeyer]

Author: Jeroen Demeyer

[jdemeyer]

Branch: u/jdemeyer/use_coercion_instead_of_pushout_in_matrix_actions

[jdemeyer]

Commit: 437c4e8

[jdemeyer]

New commits:

437c4e8

Use coercion instead of pushout for matrix actions

[tscrim]

Reviewer: Travis Scrimshaw

[tscrim]

comment:6

LGTM, but could you add a linebreak to the doctest you added? Thanks.

[sagetrac-git]

Changed commit from 437c4e8 to 9da3db6

[sagetrac-git]

Branch pushed to git repo; I updated commit sha1. New commits:

9da3db6

Add line breaks in long doctest

[vbraun]

Changed branch from u/jdemeyer/use_coercion_instead_of_pushout_in_matrix_actions to 9da3db6