deprecate expression evaluation in _parse_bound

[gdementen]

Right now, I don't see any case where f strings are not better, and this would simplify quite a bit of code. I need to be sure about this before I proceed with the deprecation though.

I don't think the feature is knowingly used by anybody, but there are surely some (many?) accidental usages out there (simply forgetting the f before the f string and not noticing because the code actually works), so we must provide a nice upgrade path/warning message and not remove the feature directly.