Add tests for sphinx 3.4 and 3.5

Author: Chilipp

.github/workflows/python-app.yml

@@ -15,7 +15,7 @@ jobs:
     strategy:
       matrix:
         python-version: [3.5, 3.6, 3.7, 3.8]
-        sphinx-version: ["", "3.2.*",  "3.1.*", "3.0.*", "2.4.*", "2.3.*", "2.2.*"]
+        sphinx-version: ["", "3.5.*", "3.4.*", "3.2.*",  "3.1.*", "3.0.*", "2.4.*", "2.3.*", "2.2.*"]
 
     steps:
     - uses: actions/checkout@v2