cmd/clef: fixups to the python clef poc (ethereum#24440)

This PR fixes up the example python clef wrapper. The poc is intended to demonstrate how to wite a UI for clef, and had severely bitrotted.
With these changes, it "works" in the sense that all the built-in tests triggers the intended python callbacks (no errors about method not found). It does not "work" in the sense that the wrapper can be used as an actual UI. It will auto-reject any signing requests, for example.

Author: holiman