move doc dependencies to pyproject

tox.ini

@@ -28,7 +28,7 @@ commands =
 description = invoke sphinx-build to build the HTML docs
 basepython = python3.11
 skip_install = True
-deps = -r{toxinidir}/docs/requirements.txt
+deps = .[docs]
 install_command = python -I -m pip install {opts} {packages}
 commands =
     sphinx-build -T -E -b html -d "{toxworkdir}/docs_doctree" -D language=en docs "{toxworkdir}/docs_out" {posargs}