plugin events: update following `Events` protoXEP submission: update the plugin to follow the specification proposed at https://github.com/xsf/xeps/pull/1206 Events are internally converted to a dict following a format described in `event_data_2_event_elt` docstring. RSVP mechanism now uses Pubsub Attachments (XEP-0470), with a user extensible data form. Bridge methods signatures have changed. rel 372

You'll find install instructions in documentation (see doc/installation.rst).

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