This truncation is already enshrined at the identifier level. Truncate
the object itself as well, to reduce the possibility multiple different
metadata objects with the same identifier.

All other hashable objects but ExtId have one.

It will be used by swh-deposit.

this object aims at being able to keep in the SWH Archive an
  SWHID <-> External object ID
map, e.g. to be able to keep track of Mercurial ids so the Mercurial
loader can be made more efficient.

Related to T2849.

was modifying the dict given as argument.

So that they can be properly deduplicated and referenced.

SWHID is deprecated; and CoreSWHID does not support qualifiers at all,
so RawExtrinsicMetadata no longer needs to check there are no qualifiers.

ExtendedSWHID can identify either a software artifact or an origin,
so we no longer need Union[SWHID, str].

Therefore, we no longer need the 'type' attribute, as it was only
used to tell when the target is a SWHID and when it's an origin URL.

It can be handy as a shortcut to build SWHID objects.

Related to T2961

The optional nature of the type will allow to avoid migrating the
(db) data model right now then we can have this type field in kafka
messages in the origin-visit-status topic

Related to T2443

Unfortunately, sphinx (actually, autodoc) only picks up attributes if
they fall in any of these cases:

1. are enum variants
2. are in slots
3. are in __dict__
4. have an annotation
5. are found using its custom parser

(see get_object_members in sphinx/ext/autodoc/importer.py)

In theory, option 5 should work for us; unfortunately, autodoc only
asks the parser the list of members with a comment.
And it's not easy to adapt it to ask the parser for all members,
because said parser (sphinx/pycode/parser.py) does not return the class
qualname (aka. namespace) for members without comments.

So, as I don't want to change the interface of sphinx.pycode.parser,
this commit switches to relying on option 3, by adding __slots__ for
all attr classes.

Additionally, this might have some performance/memory improvement
(though I did not check) and will further avoid mutation of these
objects.

All reverse dependencies have been updated to avoid using it now, so it can now
be removed, paving the way to recycle it into an intrinsic identifier.

This avoids throwing the deprecation warning when id and target are present and
have the same value, which makes a from_dict / to_dict round-trip throw no
deprecation warnings.

This backwards-compatible change prepares the transition to give
RawExtrinsicMetadata an `id` field that is computed intrinsically from its
contents (using the HashableObject mixin).

This will enable a gradual enhancement of the functions in identifiers.py to
take model objects directly, and return the bytes of the hash instead of an
hex representation.

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`.

Use the new SWHID naming convention instead of SWH PID.

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.

Related to T645

Add missing object_type class attributes on MetadataAuthority, MetadataFetcher, and RawExtrinsicMetadata.

D3560

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.

Related to T2310

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.

Related to T2310

Related to T2310

And adapt the revisions_d() strategy accordingly.

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

This requires attrs_strict >= 0.0.7.

this attribute is not an intrinsic property of a content object, so it
should not be used when comparing or hashing.

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.