Update GitHub Action to Python 3.11a6

Author: vstinner

.github/workflows/tests.yml

@@ -25,7 +25,7 @@ jobs:
     runs-on: ${{ matrix.os }}
     strategy:
       matrix:
-        python-version: [2.7, 3.5, 3.6, 3.7, 3.8, 3.9, "3.10", "3.11.0-alpha.4"]
+        python-version: [2.7, 3.5, 3.6, 3.7, 3.8, 3.9, "3.10", "3.11.0-alpha.6"]
         os: [ubuntu-latest, macos-latest]
     steps:
     - uses: actions/checkout@v2