python ci: update macos version to latest (#5406)

from errors noted in #5405 ; macos-12 has been deprecated
actions/runner-images#10721

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

@@ -154,7 +154,7 @@ jobs:
         include:
           - os: 'ubuntu-latest'
             python: '3.8'
-          - os: 'macos-13'
+          - os: 'macos-latest'
             python: '3.9'
           - os: 'windows-latest'
             python: '3.10'
@@ -217,7 +217,7 @@ jobs:
         working-directory: ${{ env.special-working-directory }}/${{ env.PROJECT_DIR}}
     strategy:
       matrix:
-        os: [ubuntu-latest, macos-12, windows-latest]
+        os: [ubuntu-latest, macos-latest, 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
@@ -327,6 +327,12 @@ jobs:
             & ".venv/bin/python" ./build/ci/addEnvPath.py ${{ env.PYTHON_VIRTUAL_ENVS_LOCATION }} venvPath
           }
 
+      - name: Install miniconda
+        uses: conda-incubator/setup-miniconda@v3
+        if: matrix.os == 'macos-latest' && matrix.test-suite == 'venv'
+        with:
+          python-version: "3.11"
+
       - name: Prepare conda for venv tests
         env:
           TEST_FILES_SUFFIX: testvirtualenvs