ci: Test all Python versions on Windows weekly

We should test pip across our entire Python support matrix on Windows,
but we don't want to run the test suite across every Python version
on very run as it's slow and unlikely to undercover a bug that the
boundary jobs won't.

.github/workflows/ci.yml

@@ -173,12 +173,21 @@ jobs:
         os: [Windows]
         python:
           - "3.8"
+          - "3.9"
+          - "3.10"
+          - "3.11"
+          - "3.12"
           - "3.13"
         group:
           - { number: 1, pytest-filter: "not test_install" }
           - { number: 2, pytest-filter: "test_install" }
-        include:
-          - python: ["3.9", "3.10", "3.11", "3.12"]
+        scheduled: ${{ github.event_name == "schedule" }}
+        exclude:
+          # Only run Windows CI across all Python versions on a weekly basis.
+          - { python: "3.9", scheduled: false }
+          - { python: "3.10", scheduled: false }
+          - { python: "3.11", scheduled: false }
+          - { python: "3.12", scheduled: false }
 
     steps:
       # The D: drive is significantly faster than the system C: drive.