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Seems more sensible if our transport format is JSON anyway. The previous
behaviour is kept for EDI compat of course.

paho seems to not really support multiple callbacks per topic, it does
not error but only the last registered callback ever gets the messages.
We could multiplex for the user, but since the case of actually wanting
multiple handlers for the same topic is probably low, just throw an
error for now.

Turns out "inspect" could do this all along. The error messages might be
slightly less helpful now, but the matching is more reliable now (it
would sometimes not count kwargs before).

This was not necessary when the callback was called from within the
try-block, but since it has been moved out of that block (so that we
don't handle exceptions thrown by the callback _itself_), the library
would continue to call the callback even when it shouldn't.

Otherwise if some command returns a number it will also be put as a
number type in the JSON. Funnily enough the matrix-bot _accepts_
non-strings as a message and passes them on to the matrix server without
error, at which point the server just throws the message away.

While this library is explicitly meant to be usable _without_ a virtual
environment of any kind, I use one to test it against python 3.6.