doc build fail on main

SciPy deprecation warning, coming via dipy.  I got started on a fix here: https://github.com/dipy/dipy/pull/3581

Here's the CI log https://app.circleci.com/pipelines/github/mne-tools/mne-python/27851/workflows/5aba661e-b142-486c-b14e-aac493ab7986/jobs/74022