[Dependabot] Add Dependabot (#13278)

Co-authored-by: Ian <ian.rodney@gmail.com>

.github/dependabot.yml

@@ -0,0 +1,21 @@
+version: 2
+updates:
+  # Tune/SGD/Doc requirements
+  - package-ecosystem: "pip"
+    # The requirements base directory currently only contains tune requirements.
+    # If we want to add more requirements here (Core, RLlib, etc.), then we should make subdirectories for each one.
+    directory: "/python/requirements"
+    schedule:
+      # TODO(amogkam) change this to weekly after some initial validation.
+      interval: "daily"
+      # 8 PM
+      time: "20:00"
+      # Use Pacific Standard Time
+      timezone: "America/Los_Angeles"
+    commit-message:
+      prefix: "[tune]"
+      include: "scope"
+    # Only 3 upgrade PRs at a time.
+    open-pull-requests-limit: 3
+    reviewers:
+      - "ray-project/ray-tune"