Have Dependabot update smmap submodule dependency

This makes Dependabot open version update PRs for submodules
(which here is just smmap), as well as GitHub Actions. This is
like gitpython-developers/GitPython#1702.

.github/dependabot.yml

@@ -3,4 +3,9 @@ updates:
 - package-ecosystem: "github-actions"
   directory: "/"
   schedule:
-      interval: "weekly"
+    interval: "weekly"
+
+- package-ecosystem: "gitsubmodule"
+  directory: "/"
+  schedule:
+    interval: "monthly"