Experimental release of python `fluent.runtime`

I wanted to add some detail on the most recent 0.3.0 release of fluent.runtime for python.

This release drastically changes the API surface. Not only does it change the API on fluent.runtime.FluentBundle, but it also introduces an API that’s actually targeting app developers. That API is fluent.runtime.FluentLocalization.

That API level doesn’t exist in the js world of @fluent/runtime, but exists in all bindings we created. On the Rust side, it’s called fluent-fallback. It’s not really a new concept, but it’s one that we put in different places under different names.

I’d like to acknowledge that Luke’s not convinced yet that this is the right direction for the API of fluent.runtime. That’s why it’s currently on a branch.

At Mozilla, we’re starting to use this API for bedrock, and we’re going to gather some experience on how to use it first hand.