GitHub Actions: Add Python 3.9 and 3.10

https://pythoninsider.blogspot.com/2021/10/python-3100-is-available.html

"3.10" ___must___ be quoted in YAML. https://dev.to/hugovk/the-python-3-1-problem-85g

Author: cclauss

.github/workflows/run-tests.yml

@@ -9,7 +9,7 @@ jobs:
     strategy:
       matrix:
         os: [ubuntu-latest]
-        python-version: [2.7, 3.5, 3.6, 3.7, 3.8, 3.9]
+        python-version: [2.7, 3.5, 3.6, 3.7, 3.8, 3.9, "3.10"]
 
     steps:
     - uses: actions/checkout@v2