use PYTHON_VERSION variable consistently

Author: jmalkin

.github/workflows/build_wheels.yml

@@ -75,10 +75,10 @@ jobs:
       - name: Checkout
         uses: actions/checkout@v4
 
-      - name: Set up Python 3.x
+      - name: Set up Python ${{ env.PYTHON_VERSION }}
         uses: actions/setup-python@v5
         with:
-          python-version: '3.x'
+          python-version: ${{ env.PYTHON_VERSION }}
 
       - name: Set up QEMU for linux/arm64 builds
         if: runner.os == 'Linux' && matrix.config.use-qemu == true