Closes T2379

Make it possible to generate Content and SkippedContent under different
object types (namely "content" and "skipped_content").

Default to False to keep backward compat.

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

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

Related to T2367

and user required=True to check that, as it is the preferred way

hat tip to @rdicosmo for noticing

This also adapts the hypothesis strategies, using the plural form
origin_visit_statuses. That plural form is acceptable because in our context,
the statuses are countable.

Related to T2310

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

Hinting at the origin_visit_update model

Related to T2310

A release may have an empty message, for instance those derived from
a Mercurial repository.

So make that field optional to avoid type validation errors.

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.

instead of a reference to an Origin entity.

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.

- add a validator for negative_utc (can be True iff offset is 0),
- update the timestamps_with_timezone hypothesis strategy,
- add low-level tests for it.

This ensures all instanciated model entities have valid types for attributes.

Related to T2308.

(pairing with @vlorentz)

Related to T2310

These are expected to be bytes, not str.

this does not work in the general case since there is no (recursive)
convertion of objects used as model object initialization.

We can only check when using the from_dict() factory.