[gh actions] Add Python 3.10 builds

Author: mpsonntag

.github/workflows/run-tests.yml

@@ -13,7 +13,7 @@ jobs:
     strategy:
       matrix:
         os: [ubuntu-latest]
-        python-version: [3.6, 3.7, 3.8, 3.9]
+        python-version: ["3.6", "3.7", "3.8", "3.9", "3.10"]
     steps:
       - uses: actions/checkout@v2
       - name: Setup Python ${{ matrix.python-version }}
@@ -33,7 +33,7 @@ jobs:
     strategy:
       matrix:
         os: [macos-latest]
-        python-version: [3.6, 3.7, 3.8, 3.9]
+        python-version: ["3.6", "3.7", "3.8", "3.9", "3.10"]
     steps:
       - uses: actions/checkout@v2
       - name: Setup Python ${{ matrix.python-version }}
@@ -54,7 +54,7 @@ jobs:
     strategy:
       matrix:
         os: [windows-latest]
-        python-version: [3.6, 3.7, 3.8, 3.9]
+        python-version: ["3.6", "3.7", "3.8", "3.9", "3.10"]
     steps:
       - uses: actions/checkout@v2
       - name: Setup Python ${{ matrix.python-version }}