Python dependency issues (2025)

Recent new dependency versions have triggered compatibility issues in `cellxgene_census` and builder. We've pinned older versions for now (#1417), pending further investigation/resolution. * New `spatialdata` and `xarray` versions triggered pip backtracking spirals involving `scipy`, `scikit-learn`, and `patsy`. (Had to pin both) * `pyarrow` 21 has a subtle API change switching `to_scipy()` return values to a new `csr_array` type, instead of a one-row `csr_matrix`, where appropriate. This broke some `cellxgene_census` unit tests, and those unit test fails might be masking other TBD issues. See https://github.com/chanzuckerberg/cellxgene-census/pull/1417#pullrequestreview-3145471914