Related to T2367

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

By providing a refresh token, obtained with the "swh auth login" command
to the new authenticate method of the client, all emitted requests will
be authenticated using a bearer token sent in headers.

Related to T1927

Usage: swh auth [OPTIONS] COMMAND [ARGS]...

  Authenticate Software Heritage users with OpenID Connect.

  This CLI tool eases the retrieval of bearer tokens to authenticate a user
  querying the Software Heritage Web API.

Options:
  --oidc-server-url TEXT  URL of OpenID Connect server (default to
                          "https://auth.softwareheritage.org/auth/")
  --realm-name TEXT       Name of the OpenID Connect authentication realm
                          (default to "SoftwareHeritage")
  --client-id TEXT        OpenID Connect client identifier in the realm
                          (default to "swh-web")
  -h, --help              Show this message and exit.

Commands:
  login    Login and create new offline OpenID Connect session.
  logout   Logout from an offline OpenID Connect session.
  refresh  Refresh an offline OpenID Connect session.

Closes T2268

Sphinx directives are not accepted as descriptions on PyPI, so we should remove
them from README. To that end, refactor README as a doc fragmented which is
linked from root dir (pointing to docs/README.rst) and included into
docs/index.rst.

intead of README.md, now gone

Closes T2279

for both tests and production