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

Note that I am not using a copyright year range here, on purpose.
As the template will be used when creating new projects, they will
generally only have copyright code starting the year of creation,
not dating back to the year of Software Heritage bootstrap. If
adaptation is needed, it should be done on a case-by-case basis.

It looks like v1.4 of Contributor Covenant has been updated upstream without
bumping version number (sigh).

so setup.py does not crash.

integrating here updates made to tox.ini in other repos, for future use

Inspired from swh-lister's dependency declaration

The initial bootstrap phase following the initialization of a new
repository is ambiguous. Adding examples should clarify this step.

with appropriate copyright/license notice in header