ci: check 3.9 on macOS

Signed-off-by: Henry Schreiner <[email protected]>

Author: henryiii

.github/workflows/ci.yml

@@ -73,6 +73,9 @@ jobs:
           - python-version: "3.9"
             runs-on: ubuntu-latest
             cmake-version: "3.20.x"
+          - python-version: "3.9"
+            runs-on: macos-latest
+            cmake-version: "3.18.x"
           - python-version: "3.10"
             runs-on: ubuntu-latest
             cmake-version: "3.22.x"