Add python3.5 to tests

Even though 3.5 is EOL'ed, there's no reason to deliberately
break it. As soon as there's a real use-case where >3.5 features
are useful, it will be removed, though.

Author: ojarva

.github/workflows/python-tests.yml

@@ -8,7 +8,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        python-version: [3.6, 3.7, 3.8]
+        python-version: [3.5, 3.6, 3.7, 3.8]
 
     steps:
     - uses: actions/checkout@v2