Pin macOS version for GHA (#133)

Co-authored-by: repo-helper[bot] <74742576+repo-helper[bot]@users.noreply.github.com>

Author: repo-helper[bot]

.github/workflows/python_ci_macos.yml

@@ -18,8 +18,8 @@ permissions:
 
 jobs:
   tests:
-    name: "macos-latest / Python ${{ matrix.config.python-version }}"
-    runs-on: "macos-latest"
+    name: "macos-13 / Python ${{ matrix.config.python-version }}"
+    runs-on: "macos-13"
     continue-on-error: ${{ matrix.config.experimental }}
     env:
       USING_COVERAGE: '3.6,3.7,3.8,3.9,3.10,3.11,3.12,3.13.0-alpha.6,pypy-3.7,pypy-3.8'