use explicit macos version

macos-latest changed to align to macos-14 - so it's no longer capable of running python 3.9

.github/workflows/ci.yml

@@ -129,7 +129,7 @@ jobs:
     runs-on: ${{ matrix.os }}
     strategy:
       matrix:
-        os: [ "macos-latest", "macos-13", "macos-14" ]
+        os: [ "macos-12", "macos-13", "macos-14" ]
         python-version: ["3.9", "3.10", "3.11", "3.12"]
         exclude:
           - os: "macos-14"