Use Python 3.13 on macOS x86_64 runner

Author: khaeru

.github/workflows/pytest.yaml

@@ -99,7 +99,7 @@ jobs:
         # Minimum version given in pyproject.toml + earlier version of Python
         # For this job only, the oldest version of Python supported by message-ix-models
         - { upstream: v3.8.0,  python: "3.10"  }  # Released 2024-01-12
-        - { upstream: v3.9.0,  python: "3.14" }  # 2024-06-04
+        - { upstream: v3.9.0,  python: "3.13" }  # 2024-06-04
         - { upstream: v3.10.0, python: "3.14" }  # 2025-02-21
         # Latest released version + latest released Python
         - { upstream: v3.11.0, python: "3.14" }  # 2025-05-27