All storage defines one endpoint even if it's mostly noop. This avoids
introspection surprises.

Related to D2966 (to be consistent)

This also adds the endpoint as noop for the main backend implementations.

Related to T2352

- blackify all the python files,
- enable black in pre-commit,
- add a black tox environment.

instead of a dict, to comply with next version of swh.journal (which will
require swh.model objects).

'swh_content_add' tries to avoid this issue with a DISTINCT clause
on the entire row; but it is useless because 'ctime' cells differ by
a few microseconds.
This commit ensures all ctime values are exactly the same, so they
are filtered out.

An alternative would be to change 'swh_content_add' to do:

```
select distinct on (sha1, sha1_git, sha256, blake2s256, length, status) sha1, sha1_git, sha256, blake2s256, length, status, ctime from tmp_content
```

instead of:

```
select distinct sha1, sha1_git, sha256, blake2s256, length, status, ctime from tmp_content
```

but this is more verbose and there's no good reason to call 'now()' for
every row.

This is required to be able to activate type validation in the model (in
swh.model.model).

It requires to replace the "distorded" usage that was done of model entities
to build objects compatible with CqlRunner's object addition logic.
Since we cannot create invalid model entities any more in this context,
we add a new CassObject type (just a dict with __getattr__=__getitem__)
and use it as object passed to the CqlRunner for entity types that need
special care (namely Revision and Release).

This should still work with swh.model v0.0.62 (without type validation) as well
as the (next) v0.0.63 which will come with type valdation.

This is not needed any more with properly typed test data.

according to model declaration, a timestamp must be a dict with 2 keys,
'seconds' and 'microseconds'.

Also add a few more tests for the date_to_db helper function so that the
test coverage of this later remains.

Related to T2310

Beside making the tests fail, they are not helpful right now. This commit will avoid the
current master build from breaking.

Also, the model bump is a mandatory preparatory work for making the origin visit
immutable.

Related to T2310

This also adds an unused (yet) optional parameter date. It will soon be used in the
context of origin_visit_update use.

Related to T2310

Reuse a date_visit from the sample storage

date argument is expected to be a datetime.

by embeding a string representation of the original Exception as
StorageArgumentException args instead of the original exc.args
since this can contain any python (possibli non-serializable) object.

This is needed e.g. when swh.model has runtime type validation.

instead of plain Content.

these are currently accepted by swh.model, but won't be any more as soon as we
activate type validation in swh.model.

This actually aligns with the origin_visit model whose status is already
mandatory.

Origin model no longer allows to have a type.

Related to f533f62bbf114cfcc29f7c72307c4dfbe99cf048

For consistency with the rest of the API.