Fix CI for macos-latest supported Python versions

Python 3.6-3.9 is no longer supported on `macos-latest`, which was recently updated to `macos-14-arm64` (see actions/setup-python#850 (comment)).

- Target macOS runner platforms by supported Python version
- Simplify specification of Ubuntu runners by moving `ubuntu-20.04` to an `include` statement

.github/workflows/main.yml

@@ -22,24 +22,33 @@ jobs:
     runs-on: ${{ matrix.os }}
     strategy:
       matrix:
-        os: [ubuntu-20.04, ubuntu-latest, macos-latest, windows-latest]
-        python-version: ["3.6", "3.7", "3.8", "3.9", "3.10", "3.11"]
+        os: [ubuntu-latest, macos-latest, windows-latest]
+        python-version: ["3.6", "3.7", "3.8", "3.9", "3.10", "3.11", "3.12"]
         dependencies:
           - pinned
           - fresh
         exclude:
-          - os: ubuntu-20.04
+          - os: ubuntu-latest
+            python-version: 3.6
+          - os: macos-latest
+            python-version: 3.6
+          - os: macos-latest
             python-version: 3.7
-          - os: ubuntu-20.04
+          - os: macos-latest
             python-version: 3.8
-          - os: ubuntu-20.04
+          - os: macos-latest
             python-version: 3.9
+        include:
           - os: ubuntu-20.04
-            python-version: "3.10"
-          - os: ubuntu-20.04
-            python-version: "3.11"
-          - os: ubuntu-latest
             python-version: 3.6
+          - os: macos-13
+            python-version: 3.6
+          - os: macos-13
+            python-version: 3.7
+          - os: macos-13
+            python-version: 3.8
+          - os: macos-13
+            python-version: 3.9
 
     env:
       # set requirements path based on Python version