use makefile via tox

docs/Makefile

@@ -20,10 +20,15 @@ endif
 help:
 	@$(SPHINXBUILD) -M help "$(SOURCEDIR)" "$(BUILDDIR)" $(SPHINXOPTS) $(O)
 
+postprocess:
+	@echo "Running postprocessing script..."
+	python ./postprocess.py
+
 clean:
 	rm -rf $(BUILDDIR)/* $(AUTODOCDIR)
 
 # Catch-all target: route all unknown targets to Sphinx using the new
 # "make mode" option.  $(O) is meant as a shortcut for $(SPHINXOPTS).
 %: Makefile
 	@$(SPHINXBUILD) -M $@ "$(SOURCEDIR)" "$(BUILDDIR)" $(SPHINXOPTS) $(O)
+	@$(MAKE) postprocess

tox.ini

@@ -56,6 +56,7 @@ commands =
 
 
 [testenv:{docs,doctests,linkcheck}]
+allowlist_externals = make
 description =
     docs: Invoke sphinx-build to build the docs
     doctests: Invoke sphinx-build to run doctests
@@ -72,8 +73,7 @@ deps =
     -r {toxinidir}/docs/requirements.txt
     # ^  requirements.txt shared with Read The Docs
 commands =
-    sphinx-build --color -b {env:BUILD} -d "{env:BUILDDIR}/doctrees" "{env:DOCSDIR}" "{env:BUILDDIR}/{env:BUILD}" {posargs}
-    python docs/postprocess.py
+    make -C {toxinidir}/docs html
 
 [testenv:publish]
 description =