run-tests.yml: add python 3.11 to matrix.

Python 3.11 is the latest stable version; add it to the GitHub test
matrix.

Author: Benjamin Moody

.github/workflows/run-tests.yml

@@ -17,7 +17,7 @@ jobs:
     strategy:
       matrix:
         os: [windows-latest, ubuntu-latest, macos-latest]
-        python-version: ["3.8", "3.9", "3.10"]
+        python-version: ["3.8", "3.9", "3.10", "3.11"]
     steps:
       - uses: actions/checkout@v2
       - name: Set up Python ${{ matrix.python-version }}