ci: Add dependabot scans for Python deps (#1618)

# What does this PR do?

This PR adds dependabot updates for Python dependencies. In addition:
* Consistent weekly schedule on a specific day
* Specific commit messages
* `open-pull-requests-limit` is intentional to avoid upgrading
dependencies that will likely cause regressions. We want to keep the
focus here on security updates only

Signed-off-by: Yuan Tang <terrytangyuan@gmail.com>

.github/dependabot.yml

@@ -5,4 +5,19 @@ updates:
   - package-ecosystem: "github-actions"
     directory: "/" # Will use the default workflow location of `.github/workflows`
     schedule:
-      interval: "daily"
+      interval: "weekly"
+      day: "saturday"
+    commit-message:
+      prefix: chore(github-deps)
+  - package-ecosystem: "uv"
+    directory: "/"
+    schedule:
+      interval: "weekly"
+      day: "saturday"
+    # ignore all non-security updates: https://docs.github.com/en/code-security/dependabot/dependabot-version-updates/configuration-options-for-the-dependabot.yml-file#open-pull-requests-limit
+    open-pull-requests-limit: 0
+    labels:
+      - type/dependencies
+      - python
+    commit-message:
+      prefix: chore(python-deps)