CI: pin py310-dev version to alpha 7 (#41315)

Author: fangchenli

.github/workflows/python-dev.yml

@@ -22,7 +22,7 @@ jobs:
     - name: Set up Python Dev Version
       uses: actions/setup-python@v2
       with:
-        python-version: '3.10-dev'
+        python-version: '3.10.0-alpha.7'
 
     - name: Install dependencies
       run: |