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Turns out there isn't an API for setting the handler, at least not a 
public one. But PythonLauncher now does test that it is the 
current handler, and shows a (supressible) warning dialog if it 
isn't.

I hope the magic of the installer, combined with the fact that 
PythonLancher advertises itself as being able to handle Python 
documents, will do the trick. I'm leaving this report open until 
that has been confirmed, though.