chore(dependabot): use deps commit type

By default, Dependabot creates commits with type "chore". Changed the
type to "deps" which is the correct type for dependencies in our
projects. "chore" was particularly problematic because it doesn't cause
release-please to create a new release.

.github/dependabot.yml

@@ -0,0 +1,8 @@
+version: 2
+updates:
+  - package-ecosystem: "pip"
+    directory: "/"
+    schedule:
+      interval: "daily"
+    commit-message:
+      prefix: "deps"