Because `setup.py` is deprecated, let's switch from:
python setup.py build_sphinx
to:
make -C docs html
to build Sphinx documentation.
The generated HTML files in `docs/_build/html` are exactly the same (I
compared with `diff -qr`).
Also add `-W` (turn warnings into errors) to the `sphinx-build` options
to keep the previous behavior.
16eae28a50
