Tox: run Sphinx docs on Python 3.7.

See also the closed PR #455 where I try a few Python versions and add some comments.

.travis.yml

@@ -12,7 +12,7 @@ matrix:
   include:
       - python: "3.8"
         env: TOXENV=isort,lint
-      - python: "3.8"
+      - python: "3.7"
         env: TOXENV=docs
       - python: "2.7"
         env: TOXENV=py27-plone-5.2

tox.ini

@@ -139,6 +139,9 @@ whitelist_externals =
     mkdir
 
 [testenv:docs]
+# Locally for Maurits this only works with Python 2.7.
+# Travis is happy with 3.7, not with 3.8 or 3.9.
+# So pick the right one in .travis.yml (or GitHub Actions).
 basepython = python
 skip_install = False
 usedevelop = True