ci: Test all Python versions on Windows on scheduled runs

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

@@ -172,17 +172,23 @@ jobs:
       matrix:
         os: [Windows]
         python:
+          # NOTE: don't forget to update middle versions below!
           - "3.8"
-          # Commented out, since Windows tests are expensively slow,
-          # only test the oldest and newest Python supported by pip
-          # - "3.9"
-          # - "3.10"
-          # - "3.11"
-          # - "3.12"
+          - "3.9"
+          - "3.10"
+          - "3.11"
+          - "3.12"
           - "3.13"
         group:
           - { number: 1, pytest-filter: "not test_install" }
           - { number: 2, pytest-filter: "test_install" }
+        scheduled: ${{ github.event_name == 'schedule' }}
+        exclude:
+          # Only run Windows CI across all Python versions during a scheduled run.
+          - { 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.