add python 3.10 to matrix

Author: satra

.github/workflows/pythonpackage.yml

@@ -16,7 +16,7 @@ jobs:
  strategy:
  matrix:
  os: [macos-latest, ubuntu-18.04]
- python-version: [3.7, 3.8, 3.9]
+ python-version: [3.7, 3.8, 3.9, "3.10"]
 
  steps:
  - uses: actions/checkout@v2