bpo-45618: Update Sphinx version used to build the documentation to 4.2.0 (GH-29256)

Updating version of Sphinx to most recent one, 4.2.0. Documentation builds without warnings.

Sphinx 4.2.0 release notes: https://www.sphinx-doc.org/en/master/changes.html#release-4-2-0-released-sep-12-2021.

cc @JulienPalard

Automerge-Triggered-By: GH:JulienPalard