Related to T2367

This allows overloading the subscribe() function, e.g for manual
assignment of partitions.

The previous implementation of this test only checked keys that were plain
bytes. It turns out that the "compounded" keys decoding was wrong.

This allows us to simplify the TEST_OBJECT_DICTS structure to only contain a
list of object dicts.

Use the opportunity to add some trivial smoke tests

This refactors the various conversions happening in the journal writer tests in
a single place.

When developing this, I noticed that the journal writer would let Content data
go through; Make sure we do not let it do that.

This matches the current configuration everywhere, and the future configuration
of brokers.

This allows us to pass objects typed as lists of ModelObject subtypes.

- blackify all the python files,
- enable black in pre-commit,
- add a black tox environment.

To test the behavior of swh.journal under different settings of the kafka
broker, split the kafka_server fixture into:

 - a persistent session_kafka_server fixture, spawning a single broker for the
 whole test session

 - a function-scoped kafka_server fixture, which uses the kafka admin client to
 override broker settings if needed, for the current test (and restores the
 original setting values afterwards)

The broker settings can be overridden for a given function using the parametrize
decorator to change the return value of the kafka_server_config_overrides
fixture.

InMemoryJournalWriter.write_addition() is expected to receive a (frozen) model
entity (not a dict) so it makes no sense any more to deepcopy it.

This matches a potential future max message size in the production kafka
clusters.

This makes the consumer loop easier to reuse in other more specific tests

And annotate the code as we go.

- Enforce installation of pyflakes from upstream git repo.
- Update pre-commit config to use a 'local' configuration.
  This is needed to be able to use flake8 with pyflakes > 2.1.1
  to gain support for @overload on methods.

This also makes it easier to be compatible with current version of swh.model
and the next version (with runtime type validation in the model).

to be compatible with the next "strongly" typed version of swh.model.

Related to 8ce4c0ba

This allows the clients to be written properly and their tests to pass.

origin model no longer allows to have type.

Related to f533f62bbf114cfcc29f7c72307c4dfbe99cf048

This will allow other journal clients to use the fixers, instead of this
being solely a replayer feature.

One could also imagine integrating the fixer directly in the journal so
that the fixes happen transparently for all the users without having to
call the fixer explicitely, and this diff is a first step in this
direction.

This aligns with what's done for the other object types.

Also:
- Add missing test cases around existing scenario
- Clarify intent about stopping the replayer when an old origin_visit format is
detected.

It crashes when the dict is converted to an swh-model object.

Related to D2813
Related to D2820