Add a github cron job

We just got bit by a bug caused by a new release of photutils. Adding
a cron job, which will include at least one test with the latest version
of dependencies, should help catch this kind of thing in the future.

.github/workflows/test.yml

@@ -6,6 +6,9 @@ on:
   push:
     branches:
       - main
+  schedule:
+    # run every Monday at 6am UTC
+    - cron: '0 6 * * 1'
 
 concurrency:
   group: ${{ github.workflow }}-${{ github.ref }}