run-tests.yml: run tests on python 3.9 and 3.10.

Expand the set of automated tests to include the newest Python
versions that GitHub supports.

Author: Benjamin Moody

.github/workflows/run-tests.yml

@@ -18,7 +18,7 @@ jobs:
     strategy:
       matrix:
         os: [windows-latest, ubuntu-latest, macos-latest]
-        python-version: [3.6, 3.7, 3.8]
+        python-version: [3.6, 3.7, 3.8, 3.9, 3.10]
     steps:
       - uses: actions/checkout@v2
       - name: Set up Python ${{ matrix.python-version }}