Related to swh/meta#4960

- 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being outdated).

This also:
- sandbox the new module into its own package to simplify packaging

Related to T4232

types-psycopg2 2.9.12 contains this commit:
https://github.com/python/typeshed/commit/506be4fb0adcf38af77325258af337772abcde77

which removes `Any` as the return type of the last variant, allowing
mypy to type-check the returned value.
However, mypy 0.920 only considers the last variant, even though we use
the first one; causing the return type to be badly inferred, and
erroring every time we use it as context manager.

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we are currently using is quite outdated and not compatible with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 also drops spurious copyright headers to those files if present.

Related to T3812

Use lowercase only for requirement names and sort them in
lexicographical order.

Move types-* in requirements-test.txt files as they are not
required for execution.

Remove no longer used requirements-test-db.txt file and associated
testing-db extra in setup.py.

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

Those db utilities were only used within swh.core. By dropping those, we can
then stop installing pifpaf within tox for the db tests.

The tests are now probably faster than before as well.

This will allow to align the Vault's client/server code with the other rpc
servers. And start adding one typed interface with more consistent checks in
between implementations.

Having tox call tox is a bit weird, but the alternatives are:
 - duplication of the test command, which is a bit sad
 - replacing all the positive factors with conjunction of negative factors, e.g.
   `core` -> `!db-!server`, which makes the logic painful to follow

One day, tox will have environment aliases which will make this horror moot :-)

(Idea pulled from a comment in https://github.com/tox-dev/tox/issues/427)

to make sure each extra setup dependencies are ok.
This requires to move test files in its related subpackage.

This class is meant to be used not only to wrap storage-like db.

Also rewrite the test_logger using pytest-postgresql fixture so
we do not need pifpaf anymore.

note that unit tests are executed via pifpaf, so a running postgres
server is not required (nor used) when unit tests are spawn with tox.

Also add .tox in gitignore.