Cap the version of dask in test environment creation

[acwooding]

The latest version of dask (2025.0.1) is not compatible with datashader (see below). Cap the version of dask in the test-requirements.txt for now.

datamapplot/__init__.py:1: in <module>
    from datamapplot.create_plots import create_plot, create_interactive_plot
datamapplot/create_plots.py:15: in <module>
    from datamapplot.plot_rendering import render_plot
datamapplot/plot_rendering.py:4: in <module>
    import datashader as ds
../../../hostedtoolcache/Python/3.10.16/x64/lib/python3.10/site-packages/datashader/__init__.py:14: in <module>
    from . import data_libraries                             # noqa (API import)
../../../hostedtoolcache/Python/3.10.16/x64/lib/python3.10/site-packages/datashader/data_libraries/__init__.py:5: in <module>
    from . import dask    # noqa (API import)
../../../hostedtoolcache/Python/3.10.16/x64/lib/python3.10/site-packages/datashader/data_libraries/dask.py:55: in <module>
    bypixel.pipeline.register(dd.core.DataFrame)(dask_pipeline)
E   AttributeError: module 'dask.dataframe.core' has no attribute 'DataFrame'

[lmcinnes]

This is also stopping docs building, so I'll have to take care of that as well I guess. Thanks for this -- it is a little annoying.

[acwooding]

Yeah that's a bit annoying. I suppose that's another reason to add tests to the docs as well.