Add Dependabot ignore directives

This adds commented out ignore directives for the following GitHub
Actions:

- action/cache
- action/checkout
- action/setup-python

These should be uncommented downstream to ensure that updates to these
dependencies are pushed from pull requests made in the skeleton.

.github/dependabot.yml

@@ -11,6 +11,10 @@ updates:
     directory: "/"
     schedule:
       interval: "weekly"
+    # ignore:
+    #   - dependency-name: actions/cache
+    #   - dependency-name: actions/checkout
+    #   - dependency-name: actions/setup-python
 
   - package-ecosystem: "pip"
     directory: "/"