Fix #569: crash when sklearn version does not collide

[janvanrijn]

fixes#569

[codecov-io]

Codecov Report

Merging #601 into develop will decrease coverage by 0.06%.
The diff coverage is 80%.

@@ Coverage Diff @@## develop #601 +/- ##
=========================================
- Coverage 90.06% 90% -0.07% 
=========================================
Files 32 32 Lines 3001 3000 -1 =========================================
- Hits 2703 2700 -3 - Misses 298 300 +2

Impacted Files	Coverage Δ
openml/flows/flow.py	95.07% <ø> (-0.17%)	⬇️
openml/flows/functions.py	93.33% <80%> (-0.86%)	⬇️
openml/flows/sklearn_converter.py	91.37% <0%> (-0.27%)	⬇️

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[mfeurer]

Does this also fix #485?

[janvanrijn]

I think #485 and this issue are duplicates. #485 is badly described though (you can actually get flows from other workbenches)