Makefile.sphinx: also exclude root tests module for apidoc as it may have content defined in its __init__.py (like in swh-web for instance)

Django wants to know the path to its configuration file at import time, which is
somewhat annoying when using autodoc on a Django project such as swh.web.

Close T763.

Works around T763 until a generic solution is found

purely cosmetic change

This allows you to still have python-sphinx and python3-sphinx coinstalled and
still build with Python 3.

... previously shipped as part of swh-environment

Closes T751

document how to publish the docs in README.md

use case: swh-storage

Before this change we were using the versionless URL
Rationale: upstream has deduplicated py2/py3 inventories.