reduce matrix load on CIs

.github/workflows/cibuildwheel.yml

@@ -67,12 +67,26 @@ jobs:
       with:
         fetch-depth: 0
         submodules: 'true'
-
+
+    - name: Build oldest and newest Python
+      shell: bash
+      # On PRs we run only oldest and newest Python versions to reduce CI load.
+      # Skips pypy and musllinux everywhere.
+      # We are buiding 38 and 312 for now.
+      # These needs to rotate every new Python release.
+      run: |
+        if [[ "${{ github.event_name }}" == "pull_request" ]]; then
+          CIBW_SKIP="pp* cp36-* cp37-* *-musllinux* cp39-* cp310-* cp311-*"
+        else
+          CIBW_SKIP="pp* cp36-* cp37-* *-musllinux*"
+        fi
+        echo "CIBW_SKIP=$CIBW_SKIP" >> $GITHUB_ENV
+        echo "Setting CIBW_SKIP=$CIBW_SKIP"
+
     - name: "Building ${{ matrix.os }} (${{ matrix.arch }}) wheels"
       uses: pypa/[email protected]
       env:
-        # Skips pypy and musllinux for now.
-        CIBW_SKIP: "pp* cp36-* cp37-* *-musllinux*"
+        CIBW_SKIP: ${{ env.CIBW_SKIP }}
         CIBW_ARCHS: ${{ matrix.arch }}
         CIBW_BUILD_FRONTEND: build
         CIBW_MANYLINUX_X86_64_IMAGE: manylinux_2_28