Allow --branch and --language to take multiple args

[JulienPalard]

So we can do:

/srv/docsbuild/venv/bin/python /srv/docsbuild/scripts/build_docs.py --quick --language en --branch 3.6 3.7 3.8

Instead of:

for branch in 3.6 3.7 3.8; do /srv/docsbuild/venv/bin/python /srv/docsbuild/scripts/build_docs.py --quick --language en 

--branch $branch; done