because we can also `cat mnt/archive/1:swh:cnt:1235...` !

This abstracts methods needed for

* folder structure creation, behind `GraphBackend`, and
* BLOB access, behind `ObjBackend`.

Former methods have been bundled in `WebApiBackend`, which implements both interfaces.

This MR also introduces a new `GraphBackend`: `CompressedGraphBackend`. It can be enabled via configuration. It's accessed and tested via its gRPC interface - tests rely directly on data in `swh.graph`.

Existing tests have been moved to `tests/graph`. I also changed the `fuse_mntdir` fixture so it starts FUSE as a thread instead of a sub-process. That way, we can see its log and exceptions' stacktraces if needed, which happens often as that FUSE process is what we're actually testing.

The implementation is as robust as possible against the variation of data available at all nodes/edges, while keeping the possibility to plug different backends.

Bump development tools: mypy, codespell, isort, ...

Move all tools configuration in pyproject.toml.

Remove no longer needed mypy overrides.

Replace pyfuse3_asyncio by pyfuse3.asyncio module introduced in pyfuse3 3.4.0

Also remove direct dependency on requests, should not be an issue any
more.

Related to swh/meta#5075.

and exclude api_data.py file from codespell in pre-commit; it's full of
stuff codespell fail to check.

`docs/images/arch-container.pdf` and `docs/images/arch-context.pdf` now
get included in `sdist` tarballs.

It now needs types-click which is indeed a dependency of
swh.core[testing].

That function from swh.core.config was removed in swh-core@2663c0a4.

GitLab will display the content of the README file when browsing the
repository. But in case the file is a symlink, it will display the
path pointed by the symlink. There is a 6 year old issue about this:
https://gitlab.com/gitlab-org/gitlab/-/issues/15093

We can workaround the issue by having the content at the root of the
repository and a symlink to this file in the `docs/` directory.

Tested in swh-py-template!27

Related to swh/meta#4960

Related to swh/meta#4959

This fixes python 3.7 support due to poetry, a dependency of isort, that
removed support for that Python version in a recent release.

In order to remove warnings about /apidoc/*.rst files being included
multiple times in toc when building full swh documentation, prefer to
include module indices only when building standalone package documentation.

Also include them the proper sphinx way.

Related to T4496

- pre-commit from 4.1.0 to 4.3.0,
- codespell from 2.2.1 to 2.2.2,
- black from 22.3.0 to 22.10.0 and
- flake8 from 4.0.1 to 5.0.4. Also freeze flake8 dependencies.

Also change flake8's repo config to github (the gitlab mirror
being outdated).

That hook can be frustrating as it can discard a long commit message
if it finds a typo in it so better removing it.

Related to T3922

black is considered stable since release 22.1.0 and the version
we are currently using is quite outdated and not compatible with
click 8.1.0, so it is time to bump it to its latest stable release.

Please note that E501 pycodestyle warning related to line length
is replaced by B950 one from flake8-bugbear as recommended by black.
https://black.readthedocs.io/en/stable/the_black_code_style/current_style.html#line-length

Related to T3922

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

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

[2]
```
06:22:34  swh fs mount:1:Unexpected indentation.
06:22:34  make: *** [../../swh-docs/Makefile.sphinx:32: sphinx/html] Error 2
```

Due to test modules being copied in subdirectories of the
build directory by setuptools, it makes pytest fail by raising
ImportPathMismatchError exceptions when invoked from root
directory of the module.

So ignore the build folder to discover tests.

To install the new hook:

  $ pre-commit install -t commit-msg

This fixes the warning about the archive.softwareheritage.org link in the markdown.

```
09:45:50  Warning, treated as error:
09:45:50  /var/lib/jenkins/workspace/DFUSE/tests-on-diff/docs/tutorial.md:85:hardcoded link 'https://archive.softwareheritage.org/api/' could be replaced by an extlink (try using ':swh_web:`api/`' instead)

```

Related to D6967

This also drops spurious copyright headers to those files if present.

Related to T3812

The target is kept so it's still usable if needed. It's dropped from the publish build
as this creates problem otherwise.

Related to T3665