extensions: deprecate extsetup without a ui argument (API)
9.5 years should be enough time, but there were some tests for the old style
still (which are now updated). Exthelper doesn't fallback to the old API, so
this is for consistency.
The extension hook ``extsetup`` without a `ui` argument has been deprecated, and will be removed in the next version. Add a `ui` argument to avoid the deprecation warning.