Improve PyPI lister to pull last update information when running incrementally

Even though the XMLRPC api for PyPI is "on the way out", it's still the recommended way of subscribing to changes for packages.

Following the instructions at https://warehouse.pypa.io/api-reference/feeds.html, it should be possible for the PyPI lister to populate a "last update" field for most listed origins. This will help us to schedule the origin visits more effectively, and will reduce the loader thrashing on origins that haven't been updated since the last visit.

From a quick test, it looks like the "Project and release activity details" feed can go back multiple years without any issue, allowing us to backfill the data for all known origins, before adding the incremental behavior to the lister.

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Migrated from T3399 (view on Phabricator)