CI: test Python 3.5 instead of 3.4 on Windows.

Fixes:

    Collecting docutils
      Could not find a version that satisfies the requirement docutils (from versions: )
    No matching distribution found for docutils

.github/workflows/ci.yml

@@ -171,7 +171,7 @@ jobs:
     strategy:
       matrix:
         include:
-        - python: '3.4'
+        - python: '3.5'
           os: windows-2019
         - python: '3.9'
           os: windows-2019