fix GHA python version setup

Author: bnavigator

.github/workflows/main-ci.yml

@@ -22,11 +22,12 @@ jobs:
         python-version: ['2.7', '3.5', '3.6', '3.7', '3.8', '3.9', '3.10', 'pypy-3.7']
 
     steps:
+      - name: Checkout source
       - uses: actions/checkout@v2
-      #    - name: Set up Python
-      #      uses: actions/setup-python@v2
-      #      with:
-      #        python-version: 3.9
+      - name: Set up Python
+        uses: actions/setup-python@v2
+          with:
+            python-version: ${{ matrix.python-version }}
       - name: Install dependencies
         run: |
           python -m pip install --upgrade pip