Add a simple alternative "client" in pure-python

It allows other components that depend on swh-graph to run their tests without depending on the WebGraph process itself.

I did not implement all the max_edges/limit stuff because they are not tested. Instead there are TODOs and a task to implement them: #3305

Test Plan

Made test fixtures parametrized, so existing tests also run on this "client"

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Migrated from D5665 (view on Phabricator)