Refines tox matrix, adds a new target "sphinxdev" that can test the l…

…atest upstream development branch

tox.ini

@@ -1,8 +1,10 @@
 [tox]
 envlist =
-    py{2,27}-sphinx{16,17,18,20,21,22,23,24,30,31,32,33,34,35,40,41,42,43,44,45,50,51}{-html4,-html5,}{-qa,}
-    py{3,36,37,38,39}-sphinx{16,17,18,20,21,22,23,24,30,31,32,33,34,35,40,41,42,43,44,45,50,51,latest}{-html4,-html5,}{-qa,}
-    py{310}-sphinx{42,43,44,45,50,51,latest}{-html4,-html5,}{-qa,}
+    # Python 2.7 support was removed in Sphinx 2
+    py{2,27}-sphinx{16,17,18}{-html4,-html5,}{-qa,}
+    py{3,36,37,38,39}-sphinx{16,17,18,20,21,22,23,24,30,31,32,33,34,35,40,41,42,43,44,45,50,51,52,latest}{-html4,-html5,}{-qa,}
+    # Python 3.10 working from Sphinx 4.2 and up
+    py{310}-sphinx{42,43,44,45,50,51,52}{-html4,-html5,latest}{-qa,}
 
 [testenv]
 setev =
@@ -35,8 +37,12 @@ deps =
     sphinx50: Sphinx>=5.0,<5.1
     sphinx51: Sphinx>=5.1,<5.2
     sphinx52: Sphinx>=5.2,<5.3
+    # All these Sphinx versions actually break since docutils 0.18, so we need to add this upper bound
+    # Projects using these Sphinx versions will have to do the same
+    # See: https://github.com/readthedocs/sphinx_rtd_theme/pull/1304
     sphinx{16,17,18,20,21,22,23,24,30,31,32,33,34,35,40,41,42}: docutils<0.18
     sphinxlatest: Sphinx
+    sphinxdev: https://github.com/sphinx-doc/sphinx/archive/refs/heads/master.zip
 commands =
     pytest {posargs} tests/
     !html4: sphinx-build -b html -Dhtml4_writer=0 -d {envtmpdir}/doctrees docs/ {envtmpdir}/html