Removing an object from Kafka requires writing a new message with
the same key as previously used and an empty value. These tombstones
then get later “compacted” depending on the topic settings `cleanup.policy`,
`max.compaction.lag.ms` and `delete.retention.ms`.

To test the presence or absence of objects in Kafka, we thus need to
find which is the most recent: a tombstone or a value. In order to do
so, we parse all messages into a single dict, associating SWHIDs with
the latest message timestamp and if it should be considered present or
absent. This is sadly a bit time and memory consuming but at least we
get accurate results.

While not strictly necessary, we now use a topic configuration in
Kafka that will aggressively try to remove “dead” messages. It
should improve slightly the time needed to inventory objects as
previously described.

We use the match syntax introduced in Python 3.10 in `handle_message()`,
so we bump black compatibility settings to Python 3.11.

Depends on swh/devel/swh-alter!7 (and a new release thereafter)