Apply dependabot to github actions.

Also issue updates on mondays and limit to 2 pull requests at a time.

Author: joaander

.github/dependabot.yml

@@ -3,7 +3,21 @@ updates:
 - package-ecosystem: pip
   directory: "/"
   schedule:
-    interval: monthly
-  open-pull-requests-limit: 10
+    interval: "weekly"
+    time: "07:00"
+    timezone: "EST5EDT"
+  open-pull-requests-limit: 2
   reviewers:
     - glotzerlab/signac-maintainers
+- package-ecosystem: "github-actions"
+  directory: "/"
+  target-branch: main
+  schedule:
+    interval: "weekly"
+    time: "07:00"
+    timezone: "EST5EDT"
+  pull-request-branch-name:
+    separator: "-"
+  reviewers:
+    - glotzerlab/signac-maintainers
+  open-pull-requests-limit: 2