.github: adapt workflow with macOS 14 arm64 runners

macos runners migrated to macOS 14 with arm64 architecture. Python 3.7
is not supported there, as GitHub reports:

    The version '3.7' with architecture 'arm64' was not found for
    macOS 14.4.1.
    The list of all available versions can be found here:
    https://raw.githubusercontent.com/actions/python-versions/main/versions-manifest.json

.github/workflows/run_tests.yml

@@ -30,7 +30,9 @@ jobs:
           - "pypy-3.9"
           - "pypy-3.10"
         exclude:
-          # macos-11 only has pypy-3.7+
+          # macos-14 with arm64 architecture only has Python 3.8+
+          - os: macos-latest
+            python-version: "3.7"
           - os: macos-latest
             python-version: pypy-3.6
           # windows-2022 only has pypy-3.7+