Expand tox config

* Adds support for explicit tox environments for html4/5
* Adds utility factors for py2/3, for easy access to the system python3
  binary
* Adds a noop -qa postfix factor to make it easy to compare against
  master or a tag

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-envlist = py{27,36,37,38,39}-sphinx{16,17,18,20,21,22,23,24,30,31,32,33,34,35,40,41,latest}
+envlist = py{2,27,3,36,37,38,39}-sphinx{16,17,18,20,21,22,23,24,30,31,32,33,34,35,40,41,latest}{-html4,-html5,}{-qa,}
 
 [testenv]
 setev =
@@ -28,4 +28,5 @@ deps =
     sphinxlatest: Sphinx
 commands =
     pytest {posargs} tests/
-    sphinx-build -b html -d {envtmpdir}/doctrees docs/ {envtmpdir}/html
+    !html4: sphinx-build -b html -Dhtml4_writer=0 -d {envtmpdir}/doctrees docs/ {envtmpdir}/html
+    html4: sphinx-build -b html -Dhtml4_writer=1 -d {envtmpdir}/doctrees docs/ {envtmpdir}/html