Added an "htmlview" target to open docs after building them.

This is helpful if you build and test documentation updates in
your development env.

Copied from CPython's Doc/Makefile.

docs/Makefile

@@ -2,6 +2,7 @@
 #
 
 # You can set these variables from the command line.
+PYTHON        ?= python
 SPHINXOPTS    ?=
 SPHINXBUILD   ?= sphinx-build
 PAPER         ?=
@@ -15,11 +16,12 @@ ALLSPHINXOPTS   = -n -d $(BUILDDIR)/doctrees -D language=$(LANGUAGE) $(PAPEROPT_
 # the i18n builder cannot share the environment and doctrees with the others
 I18NSPHINXOPTS  = $(PAPEROPT_$(PAPER)) $(SPHINXOPTS) .
 
-.PHONY: help clean html dirhtml singlehtml pickle json htmlhelp qthelp devhelp epub latex latexpdf text man changes linkcheck doctest gettext
+.PHONY: help clean html htmlview dirhtml singlehtml pickle json htmlhelp qthelp devhelp epub latex latexpdf text man changes linkcheck doctest gettext
 
 help:
 	@echo "Please use \`make <target>' where <target> is one of"
 	@echo "  html       to make standalone HTML files"
+	@echo "  htmlview   to open the index page built by the html target in your browser"
 	@echo "  dirhtml    to make HTML files named index.html in directories"
 	@echo "  singlehtml to make a single large HTML file"
 	@echo "  pickle     to make pickle files"
@@ -48,6 +50,9 @@ html:
 	@echo
 	@echo "Build finished. The HTML pages are in $(BUILDDIR)/html."
 
+htmlview: html
+	$(PYTHON) -c "import webbrowser; webbrowser.open('_build/html/index.html')"
+
 dirhtml:
 	$(SPHINXBUILD) -b dirhtml $(ALLSPHINXOPTS) $(BUILDDIR)/dirhtml
 	@echo