Setup dependabot (talonhub#953)

Setup dependabot to automatically open PRs bumping the versions of
GitHub actions used in the CI.

For instance, the `actions/setup-python` we use is out of date, which
we'll be notified of once this is merged.

.github/dependabot.yml

@@ -0,0 +1,7 @@
+version: 2
+updates:
+  # Maintain dependencies for GitHub Actions
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "daily"