Refines tox matrix

tox.ini

@@ -1,8 +1,15 @@
 [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}{-html4,-html5,}{-qa,}
+    # Python 3.10 working from Sphinx 4.2 and up
+    py{310}-sphinx{42,43,44,45,50,51}{-html4,-html5,}{-qa,}
+    # Sphinx 6 (currently latest) will drop Python 3.6 and 3.7 support
+    # https://www.sphinx-doc.org/en/master/changes.html
+    # Sphinx 7 dropping HTML 4 support, we might as well stop testing now as it's deprecated since Sphinx 5.2
+    # https://github.com/sphinx-doc/sphinx/pull/10843
+    py{38,39,310}-sphinx{latest}{-html5,}{-qa,}
 
 [testenv]
 setev =
@@ -35,6 +42,9 @@ 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
 commands =