only use tkagg or agg backends?

Nowadays the Tk backend works pretty well and is available everywhere. Maybe we should just use it (or the Agg backend for non-interactive code) and drop support for all of the other backends (which complicate the code a lot for not much benefit).

@PaulSoderlind, you mentioned wanting the Qt5 backend; could you elaborate?

No files, tests, or entry points are named. First clarify whether the project should support only Tk and Agg or also Qt5, then review how backend selection currently works. The work is complete only when the supported-backend policy is decided and its implications for existing backend support are addressed.

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