DOC: pin sphinx theme to avoid mobile dropdown bug
In the latest master version of the pydata-sphinx-theme, there is a bug in the menu dropdown on mobile mode (https://github.com/pydata/pydata-sphinx-theme/issues/304), and since this will probably not be fixed by tomorrow when releasing 1.2.2, I am pinning it here to a few commits earlier to avoid this specific bug (but still using the latest version to have the same looks as the current docs, since we are using a few features that are not yet in the latest stable release).