This introduces a new incremental behavior for the pypi lister to diminish further
existing origins without any last update.
With the incremental parameter set to False (the default for retrocompatibility), the
existing listing happens as before, this uses the simple api lists everything that's
exposed in the returned result. No last_update per origin is extracted from that
endpoint.
With the incremental parameter set to True, the listing actually only fetches the last
changes since that last visit. This also uses the XML-RPC API exposed to retrieve only
the last update for each of the origins listed. The state of the lister is then updated
to the current visit date.
Related to T3399