update pl.datatypes.UInt32 to resolve errors

[sherlyn99]

Error encountered when using polars-u64-idx v1.33.1:

  File "/home/y1weng/mambaforge/envs/micov_installation/lib/python3.12/site-packages/micov/_constants.py", line 8, in <module>
    COLUMN_START_DTYPE = pl.UInt32
                         ^^^^^^^^^
AttributeError: module 'polars' has no attribute 'UInt32'

Starting from polars-u64-idx v1.22, polars requires pl.datatypes.UInt32 instead of pl.UInt32.

[wasade]

If we do this we need to pin the allowed version of polars, can that be added to to pyproject?