When parsing the configuration, tox would complain about the unfollowed
line continuation (which is what happens when the testenv was qualified
with neither full nor minimal).

Moving {posargs} to be unqualified allows the line continuation
character to always have something behind it.

Better using latest mypy release.

Related to swh/meta#4960

See: https://tox.wiki/en/latest/upgrading.html#updating-usage-with-e

Related to swh/meta#4959

- 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being outdated).

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we are currently using is quite outdated and not compatible with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

If run after py3-full, it overwrites the full coverage info, so most lines
are incorrectly reported as uncovered in Phabricator diffs.

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

Was added in be8f1a55

Following the discussion on T3034, we decided to replace SWHID with
two or three classes:

* QualifiedSWHID to replace the existing SWHID (standard types + qualifiers)
* CoreSWHID, for "core SWHID" only (standard types + no qualifiers)
* ExtendedSWHID for internal use in Software Heritage (extra types + no qualifiers)

Since migrating from SWHID will break existing code, this commit uses
the opportunity to modernize it a little, ie.:

* `keyword`-only constructor, to get rid of the hacky default values for
  `object_type` and `object_id`
* enum instead of strings for the object type
* `bytes` instead of an hex string for the object id
* rename `metadata` to `qualifiers`

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

Allows us to call tests on things other than python3, with the same settings.

Related D2082

This prevent from requiring a too much recent tox version to fire the
tests, especially this allows to run tests using stretch's tox version.