so one can choose not to generate some of the object types.

Blacklist "origin_visit_status" by default to prevent breaking dependent
packages' tests.

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

Fix keyword parameters transmission to snapshots_d strategy.

Ensure max_size constraint is respected when fixing snapshot aliases.

With support for str representation of date.
Mostly for testing purpose.

for each entity model `Model`, provide a `models_d` strategy that
produces dicts suitable for using as argument for the `Model.from_dict`
factory method, and reimplement the `models` generator using this
former hypothesis generator.

This is needed to help writing low level tests for model entities.

But keep support for deserializing from str, like
swh-storage does.