Used webbroswer module in docs Makefile.

docs/Makefile

@@ -61,7 +61,7 @@ html:
 	@echo "Build finished. The HTML pages are in $(BUILDDIR)/html."
 
 htmlview: html
-	$(PYTHON) -c "import webbrowser; webbrowser.open('_build/html/index.html')"
+	$(PYTHON) -m webbrowser "$(BUILDDIR)/html/index.html"
 
 dirhtml:
 	$(SPHINXBUILD) -b dirhtml $(ALLSPHINXOPTS) $(BUILDDIR)/dirhtml