Use the macos-latest label rather than macos-14

Currently they are the same. The macos-latest label will move to
later versions automatically in the future, like the ubuntu-latest
and windows-latest labels that we are already using.

In this repo, the macos-14 label had been used originally because
it was added before the migration of macos-latest to be macos-14
was completed. See github/roadmap#926. It
was kept for clarity of constrast with the macos-13 jobs that were
also in use, some for the same Python versions.

Now that the macos-13 jobs have been removed in c2bbaf4, the
macos-latest label can be used here without confusion.

.github/workflows/pythonpackage.yml

@@ -13,10 +13,10 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        os: ["ubuntu-latest", "macos-14", "windows-latest"]
+        os: ["ubuntu-latest", "macos-latest", "windows-latest"]
         python-version: ["3.7", "3.8", "3.9", "3.10", "3.11", "3.12"]
         exclude:
-        - os: "macos-14"
+        - os: "macos-latest"
           python-version: "3.7"
         include:
         - experimental: false