build: run the tests once a week even if our code hasn't changed.

We use internal things that Django might change at any time, so we should run
our tests even if our code hasn't changed.

Author: nedbat

.github/workflows/tests.yml

@@ -7,6 +7,11 @@ on:
   push:
   pull_request:
   workflow_dispatch:
+  schedule:
+    # Since we test against the tip of Django development, run the tests once a
+    # week, Sundays at 6:00 UTC.
+    - cron: "0 6 * * 0"
+
 
 defaults:
   run: