Types of changes

Scipy has since 1.6 new C++ LP solvers that can handle very well sparse matrices and lead to better solutions.

This PR bump the scipy requirement and use now as default solver for LP barycenter the new "highs-ipm" solver that is more than 10 times faster on the example in the documentation

Motivation and context / Related issue

How has this been tested (if it applies)

PR checklist

• I have read the CONTRIBUTING document.
• The documentation is up-to-date with the changes I made (check build artifacts).
• All tests passed, and additional code has been covered with new tests.
• I have added the PR and Issue fix to the RELEASES.md file.