Move polars to list of required packages

[svengiegerich]

Based on my user experience, polars seem to be as it is an required and not just an optional package.

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I base this on the following error message,

  File "/home/conda/staged-recipes/build_artifacts/mrmr-selection_1715720114089/test_tmp/run_test.py", line 2, in <module>
    import mrmr
  File "/home/conda/staged-recipes/build_artifacts/mrmr-selection_1715720114089/_test_env_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_p/lib/python3.12/site-packages/mrmr/__init__.py", line 3, in <module>
    from . import polars
  File "/home/conda/staged-recipes/build_artifacts/mrmr-selection_1715720114089/_test_env_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_p/lib/python3.12/site-packages/mrmr/polars.py", line 3, in <module>
    import polars as pl
ModuleNotFoundError: No module named 'polars'

which I ran into here, trying to specify all necessary requirements for adding the package to conda.