Change defaults for get_task to be lazy

[PGijsbers]

Unfortunately, we missed a warning last patch, so it still won't close #1034

edit: letting CI run since local unit tests are slow for some reason.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 20.00000% with 8 lines in your changes missing coverage. Please review.

Project coverage is 84.02%. Comparing base (d37542b) to head (2ff431c).
Report is 1 commits behind head on develop.

Files with missing lines	Patch %	Lines
openml/tasks/functions.py	11.11%	8 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1354      +/-   ##
===========================================
- Coverage    84.12%   84.02%   -0.10%     
===========================================
  Files           38       38              
  Lines         5298     5297       -1     
===========================================
- Hits          4457     4451       -6     
- Misses         841      846       +5     

Flag	Coverage Δ
	84.02% <20.00%> (-0.10%)	⬇️

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