only check strings for new datasets

[amueller]

Follow up on #822: old datasets violate this restriction (thought I guess not the restrictions for the name?)

Does that need a test? It would maybe be easiest to roll back the schema to what it was?

[codecov-io]

Codecov Report

Merging #824 into develop will increase coverage by <.01%.
The diff coverage is 100%.

@@             Coverage Diff             @@
##           develop     #824      +/-   ##
===========================================
+ Coverage    88.05%   88.05%   +<.01%     
===========================================
  Files           36       36              
  Lines         4243     4244       +1     
===========================================
+ Hits          3736     3737       +1     
  Misses         507      507

Impacted Files	Coverage Δ
openml/datasets/dataset.py	87.5% <100%> (+0.03%)	⬆️

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