Revert "fix: make xarray downstream tests work" (#4047)

* Revert "fix: make xarray downstream tests work (#4043)"

This reverts commit b9d3964.

* ci: install only the test-py313 pixi environment for xarray downstream

With no committed pixi.lock in pydata/xarray, setup-pixi's bare
`pixi install` solves every environment in the manifest, including
`mypy-upstream`, which sources numcodecs from git and fails under newer
pixi with `meson-python: error: Unknown option "pixi-conda-environment"`
during the solve — before any test runs. Setting `environments:
test-py313` makes setup-pixi run `pixi install -e test-py313`, which
never solves that environment.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

* ci: run xarray downstream tests via uv instead of pixi

pixi has no committed lockfile in pydata/xarray, so `pixi install` solves
the entire manifest regardless of `-e <env>` — including the
`mypy-upstream` environment, which sources numcodecs from git and fails
to build under newer pixi with
`meson-python: error: Unknown option "pixi-conda-environment"`. The job
died at environment setup before any test ran, and the per-environment
flag (the previously reverted approach) does not avoid the failing solve.

Install xarray with uv from its checkout instead — the `dev` dependency
group for pytest tooling plus the `io`/`parallel`/`accel` extras for the
zarr-relevant backends — then override zarr with the branch build. Tests
needing an uninstalled backend skip via xarray's `requires_*` markers.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

Author: d-v-b

.github/workflows/downstream.yml

@@ -33,34 +33,44 @@ jobs:
           path: xarray
           persist-credentials: false
 
-      - name: Set up pixi
-        uses: prefix-dev/setup-pixi@5185adfbffb4bd703da3010310260805d89ebb11 # v0.9.6
+      # We install xarray with plain pip/uv rather than pixi. pixi solves
+      # xarray's entire manifest (it has no committed lockfile), which drags in
+      # the `mypy-upstream` environment; that environment sources numcodecs from
+      # git and fails to build under newer pixi with
+      # `meson-python: error: Unknown option "pixi-conda-environment"`, breaking
+      # the job before any test runs. Tests that need a backend we don't install
+      # are skipped via xarray's `requires_*` markers, not failed.
+      - uses: actions/setup-python@a309ff8b426b58ec0e2a45f0f869d46889d02405 # v6.2.0
         with:
-          manifest-path: xarray/pixi.toml
-          # Only install the environment our steps use. Installing every
-          # environment in xarray's manifest (the default) drags in
-          # `mypy-upstream`, which builds numcodecs from git and fails under
-          # newer pixi with `meson-python: error: Unknown option
-          # "pixi-conda-environment"` — unrelated to the zarr-python branch
-          # under test.
-          environments: test-py313
-
-      - name: Install zarr-python from branch
+          python-version: '3.13'
+
+      - name: Install uv
+        uses: astral-sh/setup-uv@08807647e7069bb48b6ef5acd8ec9567f424441b # v8.1.0
+
+      - name: Install xarray and test dependencies
         working-directory: xarray
-        run: pixi run -e test-py313 -- pip install --no-deps ..
+        run: |
+          uv venv
+          # xarray's pytest tooling lives in the PEP 735 `dev` dependency group;
+          # the zarr-relevant backends come from the `io` and `parallel` extras.
+          uv pip install --group dev ".[io,parallel,accel]"
+
+      - name: Override zarr-python with branch version
+        working-directory: xarray
+        run: uv pip install --no-deps ..
 
       - name: Show versions
         working-directory: xarray
         run: |
-          pixi run -e test-py313 -- python -c "
+          uv run python -c "
           import zarr; print(f'zarr {zarr.__version__}')
           import xarray; print(f'xarray {xarray.__version__}')
           "
 
       - name: Run xarray zarr backend tests
         working-directory: xarray
         run: |
-          pixi run -e test-py313 -- python -m pytest --no-header -q \
+          uv run python -m pytest --no-header -q \
             xarray/tests/test_backends.py \
             xarray/tests/test_backends_api.py \
             xarray/tests/test_backends_datatree.py