macos-latest should be macos-13

macos-latest has apparently been updated to point to macos-14, which gives
us an ARM runner. Avoid this for now.

.github/workflows/ci.yml

@@ -46,7 +46,7 @@ jobs:
         os: [ubuntu-latest]
         python-version: ["3.8", "3.9", "3.10", "3.11", "3.12"]
         include:
-        - os: macos-latest
+        - os: macos-13
           python-version: "3.10"
         - os: windows-latest
           python-version: "3.10"
@@ -71,7 +71,7 @@ jobs:
     timeout-minutes: 20
     strategy:
       matrix:
-        os: [ubuntu-20.04, windows-2019, macos-latest]
+        os: [ubuntu-22.04, windows-2019, macos-13]
     runs-on: ${{ matrix.os }}
     steps:
     - uses: actions/checkout@v4