this aims at preventing constant usage of isinstance() based dispatch
code when writing generic code handling model entities.

For example, the "object_type" argument of JournalWriter.write_addition() has
become superflous now we only pass model entities, etc.

This idea comes olasd's reading of mypy doc:

  https://mypy.readthedocs.io/en/latest/literal_types.html#tagged-unions

This comes with a refactoring of from_dict.DiskBackedContent to make
it *not* inherit from model.Content: object_type being Final, it cannot
be overloaded.

thus in TimestampWithTimezone.from_dict(). This is needed to help consuming
existing (invalid) messages from kafka.

Warning: tests added in this revision do not cover the whole
normalize_timestamp() function.

This is a step forward having model objects, declared as frozen, immutable.

This requires attrs_strict >= 0.0.7.

Simply add a BaseModel.anonymize() method. Default implementation returns
None, meaning the object is not anonymizable.

For Person, the method returns a Person whith hashed fullname (and unset
name and email).

For Revision and Release, the method returns an anonymized version of
the object, i.e. with instance of Person replaced by anonymized ones.

Production should only use aware datetimes.

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.

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

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.

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.

(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.

for better clarity on the code author's intention.

This will allow deduplicating code across loaders.

This lets us generate Content objects directly from a bytestring, with the
proper set of hashes auto-generated from the contents.

Will be used by loaders.

It just makes more sense that way.

eg. before this change, all leafs would be equal to each other.

They will be used by loaders, so they can deal only with
model objects, instead of having to do the same conversion themselves.

This removes the `data` and `save_path` arguments of `from_file` and
`from_disk`, as data loading is always deferred from now on.
To access it, users are now expected to either open the data files
themselves, or us `.to_model().with_data()`.

Regular files were created, as the 'mode' argument of os.mknod was missing.
However, creating devices requires root; so we can't reasonably do that in
tests.

Instead, we're using /dev/null instead of creating one.
And while we're at it, let's also use /dev/zero (which, if not handled
properly, will result in an infinite read).

It should be cheap enough to do it, and it makes tests easier.

It will be useful to loaders, as they currently load the entire
content in memory before deciding to skip it.

Can be useful to deduplicate code in swh-storage.

Related to P589

This allows for example to type:

  curl -s https://archive.softwareheritage.org/browse/content/sha1_git:64582b78792cd6c2d67d35da5a11bb80886a6409/raw/ | swh identify
  swh:1:cnt:64582b78792cd6c2d67d35da5a11bb80886a6409	-

Add support to automatically compute identifier in the following object models:
Directory, Release, Revision, Snapshot.

If the identifier is not provided as parameter, it will be computed when the model
is initialized.