Update tox build and docs

docs/requirements.txt

@@ -1,5 +1,5 @@
 # Requirements file for ReadTheDocs, check .readthedocs.yml.
 # To build the module reference correctly, make sure every external package
 # under `install_requires` in `setup.cfg` is also listed here!
-sphinx>=3.2.1
+sphinx>=4
 # sphinx_rtd_theme

setup.cfg

@@ -127,3 +127,43 @@ deps =
     pytest-sugar
 commands =
     pytest {tty:--color=yes} {posargs}
+
+[testenv:{build,clean}]
+description =
+    build: Build the package in isolation according to PEP517, see https://github.com/pypa/build
+    clean: Remove old distribution files and temporary build artifacts (./build and ./dist)
+# https://setuptools.pypa.io/en/stable/build_meta.html#how-to-use-it
+skip_install = True
+changedir = {toxinidir}
+deps =
+    build: build[virtualenv]
+passenv =
+    SETUPTOOLS_*
+commands =
+    clean: python -c 'import shutil; [shutil.rmtree(p, True) for p in ("build", "dist", "docs/_build")]'
+    clean: python -c 'import pathlib, shutil; [shutil.rmtree(p, True) for p in pathlib.Path("src").glob("*.egg-info")]'
+    build: python -m build {posargs}
+
+[testenv:{docs,doctests,linkcheck}]
+description =
+    docs: Invoke sphinx-build to build the docs
+    doctests: Invoke sphinx-build to run doctests
+    linkcheck: Check for broken links in the documentation
+passenv =
+    SETUPTOOLS_*
+setenv =
+    DOCSDIR = {toxinidir}/docs
+    BUILDDIR = {toxinidir}/docs/_build
+    docs: BUILD = html
+    doctests: BUILD = doctest
+    linkcheck: BUILD = linkcheck
+# skip_install = true
+allowlist_externals=echo
+deps =
+    -r {toxinidir}/docs/requirements.txt
+    # ^  requirements.txt shared with Read The Docs
+# See https://github.com/Kopfstein/pyscaffold-gh-pages-deploy/blob/main/action.yml
+commands =
+    echo "Docs build chain is unavailable via tox - blocked by a hard requirement of docutils in cdpcli. Use the Sphinx 'make' as an alternative."
+    exit
+    sphinx-build --color -b {env:BUILD} -d "{env:BUILDDIR}/doctrees" "{env:DOCSDIR}" "{env:BUILDDIR}/{env:BUILD}" {posargs}