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.