MAINT: Add "MAINT" tag to dependabot commit msg

I hope this will also change PR title accordingly. Also
try and change the labels to just include maintenance, since
it adds the "dependy" label for no good reason.

[skip-ci]

Author: seberg

.dependabot/config.yml

@@ -3,3 +3,7 @@ update_configs:
   - package_manager: "python"
     directory: "/"
     update_schedule: "weekly"
+    commit_message:
+      prefix: "MAINT"
+    default_labels:
+      - "03 - Maintenance"