This reverts commit a0f54362.

This means this commit removes the 'offset' and 'negative_utc'
arguments.

It also removes the 'negative_utc' attribute (which is not used
anymore), but keeps an 'offset' property, which is an alias to
'offset_minutes()'. This is only to keep this commit readable;
the next commit will remove this alias.

This allows keeping compatibility with existing users of the TimestampWithTimezone constructor

There is a regression that breaks attr.evolve() when updating attributes
that contain an attr class; which we use (eg. for Person or TimestampWithTimezone).

v21.2.0 is expected to fix the issue, but won't be released immediately:
https://github.com/python-attrs/attrs/issues/804#issuecomment-833471190

Related to T2105

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.

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

This requires attrs_strict >= 0.0.7.

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

Related to T2308.

Will be used by loaders.

It is now possible to install swh-model without dulwich (and Click, FWIW).

Users who want to use "swh identify" should "pip install swh-model[cli]".