ci: Ignore Dependabot PRs to update tests/constraints.txt (#2010)

* Ignore all dependencies that are under the domain of pip in Dependabot's package
  ecosystems to avoid automatic security PRs that target tests/constraints.txt,
  which must remain static for lower bound constraints testing.
* Add default labels and reviewers to be applied to PRs from GHA updates from Dependabot.

.github/dependabot.yml

@@ -5,3 +5,16 @@ updates:
     directory: "/"
     schedule:
       interval: "weekly"
+    labels:
+      - "github-actions"
+      - "dependencies"
+    reviewers:
+      - "matthewfeickert"
+
+  # Ignore all pip dependencies to avoid PRs to update tests/constraints.txt
+  - package-ecosystem: "pip"
+    directory: "/"
+    schedule:
+      interval: "weekly"
+    ignore:
+      - dependency-name: "*"