Use macos-13 to avoid errors setting up Python (#2879)

* Use macos-13 to avoid errors setting up Python

* Try with macos-12, the last that passed.

.github/workflows/positron-python-ci.yml

@@ -126,7 +126,9 @@ jobs:
           - os: 'ubuntu-latest'
             python: '3.8'
             pytest-version: 'pytest<8.1.1'
-          - os: 'macos-latest'
+          # TODO: go back to macos-latest once we have a stable version of Python 3.9
+          # on that image.
+          - os: 'macos-13'
             python: '3.9'
             pytest-version: 'pytest<8.1.1'
           - os: 'windows-latest'
@@ -180,7 +182,7 @@ jobs:
         include:
           - os: 'ubuntu-latest'
             python: '3.8'
-          - os: 'macos-latest'
+          - os: 'macos-13'
             python: '3.9'
           - os: 'windows-latest'
             python: '3.10'
@@ -243,7 +245,7 @@ jobs:
         working-directory: ${{ env.special-working-directory }}/${{ env.PROJECT_DIR}}
     strategy:
       matrix:
-        os: [ubuntu-latest, macos-latest, windows-latest]
+        os: [ubuntu-latest, macos-12, windows-latest]
         python: ['3.x']
         test-suite: [ts-unit, venv, single-workspace, debugger, functional, smoke]
         # TODO: Add integration tests on windows and ubuntu. This requires updating