add Python 3.10 beta

Author: jschlyter

.github/workflows/test.yml

@@ -14,6 +14,7 @@ jobs:
           - "3.7"
           - "3.8"
           - "3.9"
+          - "3.10.0-beta.1"
     steps:
       - uses: actions/checkout@v2
       - name: Set up Python ${{ matrix.python-version }}