Upgrade to MacOS 12 (#7)

Author: awvwgk

.github/workflows/tests.yml

@@ -9,7 +9,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        os: [macos-11, macos-10.15]
+        os: [macos-11, macos-12]
     runs-on: ${{ matrix.os }}
     steps:
       - name: Set up Homebrew