Add workflow to update python dependencies

Using the new opensafely-core/update-dependencies-action

.github/workflows/update-dependencies.yml

@@ -0,0 +1,19 @@
+name: Update python dependencies
+
+on:
+  workflow_dispatch:
+  schedule:
+    - cron:  "0 23 * * *"
+jobs:
+  update-dependencies:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v4
+    - uses: "opensafely-core/setup-action@v1"
+      with:
+        python-version: "3.11"
+        install-just: true
+    - uses: opensafely-core/update-dependencies-action@main
+      with:
+        update_command: "just compile-reqs -U"
+        on_changes_command: "just test-all"