Schedule test each month instead of each day

Author: philpep

.github/workflows/tox.yml

@@ -12,7 +12,7 @@ on:
     types:
       - published  # It seems that you can publish directly without creating
   schedule:
-    - cron: 1 0 * * *  # Run daily at 0:01 UTC
+    - cron: 1 0 15 * *  # Run each month on day 15 at 0:01 UTC
 
 concurrency:
   group: ${{ github.workflow }}-${{ github.event.pull_request.number || github.sha }}