Switch to compatible Petals target branch for testing

REVERT THIS!

Author: peanutfun

.github/workflows/ci.yml

@@ -74,6 +74,6 @@ jobs:
     needs: build-and-test
     with:
       core_branch: ${{ github.ref }}
-      petals_branch: develop
+      petals_branch: add-lxml-dependency  # TODO: DO NOT MERGE LIKE THIS
     permissions:
       checks: write