Also add 3.5 support in workflow.

.github/workflows/pythonpackage.yml

@@ -9,7 +9,7 @@ jobs:
     strategy:
       matrix:
         os: [ubuntu-latest, macos-latest, windows-latest]
-        python-version: [3.6, 3.7, 3.8, pypy3]
+        python-version: [3.5, 3.6, 3.7, 3.8, pypy3]
       fail-fast: false
 
     steps: