Fix: Attempt to make the CI build work on Python 3.9

Author: amotl

.github/workflows/pythonpackage.yml

@@ -6,11 +6,11 @@ jobs:
   build:
     name: Build Job
 
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     strategy:
       max-parallel: 4
       matrix:
-        python-version: [3.7, 3.8]
+        python-version: [3.7, 3.8, 3.9]
 
     steps:
     - uses: actions/checkout@v1