xfail failing model tests

[charlesbluca]

Until another version of dask-ml is released, test_iterative_and_prediction and test_show_models will fail due to the breakage discussed in dask/dask-ml#894; this PR xfails those tests to unblock CI for now until that new version is out, after which we will want to bump our dask-ml dependency.

[codecov-commenter]

Codecov Report

Merging #373 (9b8b617) into main (cf3c076) will decrease coverage by 0.13%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #373      +/-   ##
==========================================
- Coverage   95.60%   95.46%   -0.14%     
==========================================
  Files          67       67              
  Lines        2932     2932              
  Branches      546      546              
==========================================
- Hits         2803     2799       -4     
- Misses         79       83       +4     
  Partials       50       50              

Impacted Files	Coverage Δ
dask_sql/physical/rel/custom/show_models.py	53.33% <0.00%> (-26.67%)	⬇️

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