Add some comments to tox.ini

Mainly explaining the environment matrix

Author: bskinn

tox.ini

@@ -2,21 +2,35 @@
 minversion=2.0
 isolated_build=True
 envlist=
+    # Test all Python versions on latest lib versions
     py3{6,7,8,9,10,11}-sphx_latest-attrs_latest-jsch_latest
+    # Test dev Python version on current in-repo dev lib versions
     py310-sphx_dev-attrs_dev-jsch_dev
-    py310-sphx_{1_6_x,1_x,2_x,3_x,4_x,dev}-attrs_latest-jsch_latest
+    # Scan across Sphinx versions
+    py310-sphx_{1_6_x,1_x,2_x,4_x,dev}-attrs_latest-jsch_latest
+    # sphx_3_x is incompatible with py310 due to a typing import. Test on py39 instead.
+    py39-sphx_3_x-attrs_latest-jsch_latest
+    # Scan attrs versions
     py310-sphx_latest-attrs_{19_2,19_3,20_1,20_2,20_3,21_2,21_3,dev}-jsch_latest
+    # Scan jsonschema versions
     py310-sphx_latest-attrs_latest-jsch_{3_0,3_1,3_2,4_0,4_1,dev}
+    # Earliest supported Python and lib versions all together
     py36-sphx_1_6_x-attrs_19_2-jsch_3_0
+    # Spot matrix of early Python, Sphinx, attrs versions
     py3{7,8,9}-sphx_{1,2}_x-attrs_{19,20}_2-jsch_latest
+    # Test the specific Sphinx threshold cases where behavior changed
     py310-sphx_{2_3_1,2_4_0,3_2_1,3_3_0,3_4_0}-attrs_latest-jsch_latest
+    # Simple 'does the sdist install' check
     sdist_install
+    # Lints
     flake8
 
 [testenv]
 commands=
     python --version
     pip list
+    # Want the tox *matrix* to ignore warnings since it's primarily
+    # a compatibility check. The defaults for bare pytest enable -Werror
     pytest {posargs:--nonloc -Wignore}
 deps=
     sphx_1_6_x:   sphinx<1.7