Add sphinx-lint to the ci workflow and Makefile. (python#496)

* Add sphinx-lint to the Makefile.

Makefile

@@ -69,14 +69,16 @@ $(VENV)/bin/activate:
 $(VENV)/bin/sphinx-build: $(VENV)/bin/activate
 	. $(VENV)/bin/activate; python3 -m pip install sphinx python-docs-theme
 
+$(VENV)/bin/sphinx-lint: $(VENV)/bin/activate
+	. $(VENV)/bin/activate; python3 -m pip install sphinx-lint
 
 $(VENV)/bin/blurb: $(VENV)/bin/activate
 	. $(VENV)/bin/activate; python3 -m pip install blurb
 
 
 .PHONY: upgrade_venv
 upgrade_venv: $(VENV)/bin/activate ## Upgrade the venv that compiles the doc
-	. $(VENV)/bin/activate; python3 -m pip install --upgrade sphinx python-docs-theme blurb
+	. $(VENV)/bin/activate; python3 -m pip install --upgrade sphinx python-docs-theme blurb sphinx-lint
 
 
 .PHONY: progress
@@ -127,3 +129,7 @@ fuzzy: ## Find fuzzy strings
 .PHONY: rm_cpython
 rm_cpython: ## Remove cloned cpython repo
 	rm -rf $(CPYTHON_CLONE)
+
+.PHONY: lint
+lint:  $(VENV)/bin/sphinx-lint  ## Run sphinx-lint
+	$(VENV)/bin/sphinx-lint --enable default-role