incorrect deprecation messages for [M]Polynomial

[dimpase]

Is there an existing issue for this?

• I have searched the existing issues for a bug report that matches the one I want to file, without success.

Did you read the documentation and troubleshoot guide?

• I have read the documentation and troubleshoot guide

Environment

* Sage 10.1.beta4

Steps To Reproduce

sage: isinstance(x, sage.structure.element.Polynomial)
---------------------------------------------------------------------------
AttributeError ...
...
sage: isinstance(x, sage.structure.element.MPolynomial)
---------------------------------------------------------------------------
AttributeError  ...

The classes which exist are sage.rings.polynomial.polynomial_element.Polynomial resp.
sage.rings.polynomial.multi_polynomial.MPolynomial

How does this need to be fixed?

Expected Behavior

deprecation message should be correct

Actual Behavior

it is not

This was introduced in #35076

[dimpase]

@mkoeppe - any ideas? Correct the messages, or rather correct the code?

[mkoeppe]

The messages are wrong, a leftover from an earlier design