Attempt to fix the GHA build 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