docs: drop deprecated setuptools build_sphinx

Author: nejch

setup.cfg

@@ -83,7 +83,7 @@ per-file-ignores =
 
 [testenv:docs]
 deps = -r{toxinidir}/requirements-docs.txt
-commands = python setup.py build_sphinx
+commands = sphinx-build -W --keep-going -b html docs build/sphinx/html
 
 [testenv:cover]
 commands =