Use latest sphinx version to build the doc
Now that sphinxcontrib-images extension has gained support for Sphinx > 2 in its 0.9.1 release, we can remove the restriction on Sphinx version in the requirements file.
Also considering that we now rely on a virtualenv to build the doc, remove the hacks to invocate sphinx-build and sphinx-apidoc in Makefiles.
Related to #2264 (closed)
Migrated from D2616 (view on Phabricator)