Dependabot: switch to monthly frequency (#922)

Monthly means things don't get too out of date, and will be less noisy.

Docs:

https://docs.github.com/en/code-security/dependabot/dependabot-version-updates/about-dependabot-version-updates#frequency-of-dependabot-pull-requests

Author: hugovk

.github/dependabot.yml

@@ -3,5 +3,5 @@ updates:
 - package-ecosystem: pip
   directory: "/"
   schedule:
-    interval: daily
+    interval: monthly
   open-pull-requests-limit: 10