As the author of one of the new theme dependencies (`alabaster`), having a wholly unqualified requirement scared me a bit re: ability to make non-bugfix releases without downstream users' sites changing underneath them in unexpected (visual) ways without them consciously upgrading Sphinx versions.
`alabaster` uses semantic versioning so the lock reflects this.
For consistency, I also updated the other new theme dependency similarly, and made `babel` match its fellow non-theme dependencies in having an optimistic (vs semantic) version lock, but I can revert these changes if desired.
