Use macos-12 for testing Python 3.9

Author: tomwhite

.github/workflows/tests.yml

@@ -17,7 +17,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        os: ["ubuntu-latest", "macos-latest", "windows-latest"]
+        os: ["ubuntu-latest", "macos-12", "windows-latest"]
         python-version: ["3.9"]
         include:
           - os: "ubuntu-latest"