Change Dependabot pip directory to repo root

Since Dependabot will find the `requirements/` directory automatically,
and by using the repo root we avoid the unnecessary `in /requirements`
suffix on all Python Dependabot PR titles.

.github/dependabot.yml

@@ -22,7 +22,7 @@ updates:
       - "github actions"
       - "skip changelog"
   - package-ecosystem: "pip"
-    directory: "/requirements"
+    directory: "/"
     schedule:
       interval: "monthly"
     labels: