Work around khaeru/genno#171

Author: khaeru

.github/workflows/pytest.yaml

@@ -168,13 +168,15 @@ jobs:
     - name: Install packages and dependencies
       # By default, install:
       # - ixmp, message_ix: from GitHub branches/tags per matrix.version.upstream (above)
+      # - dask: work around https://github.com/khaeru/genno/issues/171
       # - other dependencies: from PyPI.
       #
       # To test against unreleased code (on `main`, or other branches for open PRs),
       # temporarily edit or add lines below as needed. DO NOT merge such changes to `main`.
       run: |
         uv pip install --upgrade \
           ${{ steps.dependencies.outputs.value }} \
+          "dask < 2025.4.0" \
           "ixmp @ git+https://github.com/iiasa/ixmp.git@${{ matrix.version.upstream }}" \
           "message-ix @ git+https://github.com/iiasa/message_ix.git@${{ matrix.version.upstream }}" \
           .[docs,tests]