We stop depending on the ListerBase implementation. The main hoop we're jumping
through is the config override mechanism in swh.lister.get_lister, as it's
really specifc to the ListerBase `override_config` argument, which is dropped in
pattern.Lister (in favor of explicit arguments at lister instantiation).
We implement a small shim in swh.lister.pattern.Lister to give
backwards-compatibility for the new pattern to get_lister.
This generic configuration override mechanism will probably be completely
removed when the configuration mechanism is reworked. We'll see.
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