Limit Dependabot scope to GitHub Actions only

Sometimes we intentionally pin specific package versions -- such as for
Pygments, Jinja2, and Markdown -- and thus automated Dependabot PRs for
those packages are spurious and unwanted.

.github/dependabot.yml

@@ -1,13 +1,8 @@
 # See https://docs.github.com/en/free-pro-team@latest/
 #  github/administering-a-repository/enabling-and-disabling-version-updates
-
+---
 version: 2
 updates:
-  - package-ecosystem: "pip"
-    directory: "/"
-    schedule:
-      interval: "monthly"
-    open-pull-requests-limit: 10
   - package-ecosystem: "github-actions"
     directory: "/"
     schedule: