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

Enable to check package documentation can be built without producing
sphinx warnings.

The sphinx environment is designed to be used in continuous integration
in order to prevent breaking documentation build when committing changes.

The sphinx-dev environment is designed to be used inside a full swh
development environment.

Related to T3258

and add a test to keep them correct.

According to the SWHID specification, it is not forbidden for a qualifier
value to contain a '=' character (for instance in origin URL).

So update parsing code to handle that special case.

Some ValidationError exceptions could not be serialized to string due
to these format errors.

Related to T3234

Otherwise, they will get an error asking them to install Click
(or Dulwich if Click installed and they use -t snapshot)

And show nice human-readable errors instead

Was added in be8f1a55

Some releases don't have authors and date fields, this case should be
checked in the tests.

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.

The rounding algorithm wasn't specified

Serializing as ISO8601 makes the hash brittle, because the database may
change the timezone silently and/or lose precision in the microseconds.

As we do not need precise timestamp, using an integer is good enough,
and is consistant with the git format.

The manifest also does not need to contain a timezone, as it only
represents the timezone of the system that fetched this metadata,
which is useless data.

So that they can be properly deduplicated and referenced.

This will be used to compute an intrisic identifier for RawExtrinsicMetadata;
which can be used for deduplication and refering to it like any other sha1_git
instead of needed to use a tuple of its fields.

Other packages don't use it anymore.

This is a useful shorthand when generating SWHIDs in tests.

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.