Inelegantly fix a problem where os-ver is empty

The experiment 3.13 Windows job recently started getting `runs-on`
set to `windows-` rather than `windows-latest` (causing it to block
indefinitely waiting on a self-hosted runner with `windows-` as a
label). I don't know why this happens, since it looks like it did
not happen before rebasing from 65ff35f to 6060afd, but it happens
when force-pushing back to that OID from before the rebase. This
usually indicates a change in the runner software (or some other
part of the GHA infrastructure), but it could be that it happened
before and went unnoticed (then looked like a failure but was
actually a cancellation?).

Author: EliahKagan

.github/workflows/pythonpackage.yml

@@ -27,6 +27,7 @@ jobs:
         - experimental: false
         - os-type: windows
           python-version: "3.13"
+          os-ver: latest  # Somehow the fallback (`- os-ver: latest` by itself) no longer covers this.
           experimental: true
 
       fail-fast: false