Fix silent, then loud failure after Tox 4 upgrade (readthedocs#9803)

Fix silent, then loud failure after Tox 4 upgrade re: tox-dev/tox#2658

Author: benjaoming

tox.ini

@@ -11,7 +11,7 @@ setenv =
     LANG=en_US.UTF-8
     LC_ALL=en_US.UTF-8
     DJANGO_SETTINGS_SKIP_LOCAL=True
-passenv = CI TRAVIS TRAVIS_* HOME
+passenv = CI,TRAVIS,TRAVIS_*,HOME
 deps =
   -r requirements/testing.txt
   debug: -r requirements/debug.txt