that returns a value suitable for unicity constraints.

Motivation:

* this is somewhat more of a model concern than a journal/kafka
  concern IMO
* this is one step toward adding support for non-model objects in
  KafkaJournalWriter

Implementation of the unique_key methods comes from
`swh.journal.serializers.object_key`.

I created one in the wrong directory and didn't see the existing one.

Related to T2610

We may unknowingly pass naive datetimes to the storage through them,
causing the underlying DB to assign them a timezone that might not match
the actual one.

It already happens in swh.model and swh.loader.package tests.

Same as for the field data, it helps for code not yet migrated to use model
object.

It allows easy conversion of Union[ImmutableDict, Dict] to ImmutableDict.

So they are truly immutable now.

This will allow swh-storage to have a signature for *_metadata_add
that is consistent with other *_add endpoints.

Add a new extra_headers attribute on Revision and use it for computing
the revision's id instead of extract it from the metadata field.

Only accept (bytes, bytes) as extra_header.

Add a post init hook to Revision to initialize this new attribute from
given metadata, if any, for bw compat.

Also amend the revision_d hyptothesis strategy to generate extra_headers.

When Software Heritage persistent identifiers were introduced, they were not
yet abbreviated as SWHIDs.

Now that abbreviation is growing adoption, rename some functions and types in
swh.model.identifiers for consistency:

  - PersistentId -> SWHID

  - persistent_identifier -> swhid

  - parse_persistent_identifier -> parse_swhid

Backward compatibility with previous naming is maintained but deprecation
warnings are introduced to encourage the use of the new names.

Numerous variables in swh.model codebase have also been renamed accordingly.

Also rework and improve documentation.

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.