fix (solvers): Add compatibility shim for xpress.Namespaces

[coroa]

Changes proposed in this Pull Request

xpress.Namespaces was added in version 9.6, so for versions below it, we add a compatibility shim.

This fixes a regression introduced in #497 .

Refer also to documentation:
Current: https://www.fico.com/fico-xpress-optimization/docs/latest/solver/optimizer/python/HTML/xpress.Namespaces.html .
And getnamelist from version 9.5 (before namespaces in 2023):
https://www.fico.com/fico-xpress-optimization/docs/dms2023-03/solver/optimizer/python/HTML/problem.getnamelist.html

Checklist

• Code changes are sufficiently documented; i.e. new functions contain docstrings and further explanations may be given in doc.
• Unit tests for new features were added (if applicable).
• A note for the release notes doc/release_notes.rst of the upcoming release is included.
• I consent to the release of this PR's code under the MIT license.