This changes the deprecated :show-nested: instruction to the :nested: one [1].
This also fixes warning about misdefined block [2]

[1] https://sphinx-click.readthedocs.io/en/latest/usage/#directive-click

[2]
```
10:38:38  Warning, treated as error:
10:38:38  /var/lib/jenkins/workspace/DMOD/tests-on-diff/docs/cli.rst:14:Literal block expected; none found.
10:38:38  make: *** [../../swh-docs/Makefile.sphinx:32: sphinx/html] Error 2
10:38:38  make: Leaving directory '/var/lib/jenkins/workspace/DMOD/tests-on-diff/docs'
10:38:38  ERROR: InvocationError for command '/usr/bin/make -I ../.tox/sphinx/src/swh-docs/swh/ -C docs' (exited with code 2)
```

committer=None happens on some malformed commits generated by old dgit
versions ; and it is possible for author=None to happen for the same reason.

these fields are computed attributes and may be removed from the backend
storage. Helps writing tests.

This will be used by swh.storage.backfill's tests.

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 should slightly reduce the memory used by dir/rev/rel objects

A future commit will remove the 'offset' attribute, and rename
'offset_bytes' to 'offset'.

Modules using the `.offset` attribute will need to switch to this new
method to keep working.

This allows keeping compatibility with existing users of the TimestampWithTimezone constructor

'offset' becomes a property instead of an attribute and constructor argument.

This also removes both from the output of `.to_dict()`.

This is step 6 of https://forge.softwareheritage.org/T3752

This will break packages that still use the constructor directly,
ie. swh-storage and swh-loader-git (and tests of swh-loader-core
and swh-loader-svn)

This is needed when recomputing the manifest of git objects with weird offsets.

blake2s and blake2b have been provided by the stdlib hashlib since
Python 3.6, and we declare 3.7 as minimum Python version supported.

1. Most objects do not need it so it's a waste of space
2. This means we just extend the existing format (some objects will
   have that key in their dict) instead of changing it (retroactively
   adding it to all objects)

This will be used to store the original manifest of 'weird' git objects,
when we cannot reasonably represent them otherwise.

Using .now() produces data that differs between xdist processes,
as files are imported after forking, and xdist requires consistent
data across processes.

It calls attr.validate() (which calls the validators), and recomputes
the hash of HashableObject instances.

A future commit will also make it check the raw_manifest attribute
when relevant

It's just simpler this way

For the sake of completeness (a future commit may depend on it).

For now it is filled from 'offset' and 'negative_utc', but it will replace them
in a future commit.

This is to simplify and add support for more 'weird' offsets we do not currently
support.

This reverts commit c525484e.

It enables to easily check if a path exists from a root directory.

Since rDMOD8d96dfedee34203a4118e48a6208ee507511590b, directory entry
names are validated in DirectoryEntry model and thus must not contain
any slash characters.

So update directory_entries_d hypothesis strategy to ensure such names
are generated to avoid flaky tests.

Enable to compute md5 sum through the hashutil.MultiHash class.

Nevertheless, md5 is not put in DEFAULT_ALGORITHMS set and must
be explicitely requested by client code.

Related to T2400

I don't know any instance of these, but there is no harm in checking them.

1. hashes are now repr()ed as `hash_to_bytes("1234...")` instead of b"\x12\x34..."`
2. SWHID objects are now repr()ed as `CoreSWHID.from_string('swh:1:...:1234...')`
   instead of `CoreSWHID(scheme='swh', version='1', object_type=..., object_id=b'\x12\x34')`
3. enums are now repr()ed as `MyEnum.NAME` instead of "<MyEnum.NAME: 'value'>`

Thanks to these three changes, using repr() on a model object now prints
a string that can be pasted directly in a `.py` file to write a new
test case.

The previous replaced attrs-strict's type validator with our own,
stricter and faster, validator.

However, the strictness can be a burden in other packages;
for example, swh-storage tests rely on it to insert dummy data that raises
exception when accessed, and it would be hard to do while using the exact
expected type.

This commit reverts the strict behavior, but keeps the performance
optimization, by always checking with type equality, but in case type
equality fails (which would raise an error before this commit), it gives
the value a 'second chance', by trying isinstance.

This means that, outside tests, isinstance should not be used at all,
or very rarely.

This reimplements attrs_strict.type_validator(), using type equality
instead of isinstance.

This makes my checksum validation script (that mostly just instantiates
model objects, computes a checksum, then discard) run twice as fast.

For consistency, as the classes are now in swhids.py

+ raise warnings

1. Add a warning
2. Move identifier/manifest documentation to git_objects.py
3. Remove all imports of that module.

Motivation:

* SWHID classes were moved to swhids.py
* manifest computation functions were moved to git_objects.py
* Only reexports and trivial wrappers of model.py remain