MRG Sklearn 0.21 compatibility and CI

[amueller]

Working on #749.

[amueller]

tests pass locally on 0.22.dev0, let's see what CI says

[codecov-io]

Codecov Report

Merging #752 into develop will decrease coverage by 0.08%.
The diff coverage is 60%.

@@ Coverage Diff @@## develop #752 +/- ##
===========================================
- Coverage 87.93% 87.84% -0.09% 
===========================================
Files 36 37 +1 Lines 4086 4082 -4 ===========================================
- Hits 3593 3586 -7 - Misses 493 496 +3

Impacted Files	Coverage Δ
openml/_backport.py	60% <60%> (ø)
openml/exceptions.py	84.37% <0%> (-9.38%)	⬇️
openml/utils.py	91.13% <0%> (-0.64%)	⬇️
openml/runs/functions.py	81.79% <0%> (-0.29%)	⬇️
openml/flows/flow.py	93.8% <0%> (-0.06%)	⬇️
openml/testing.py	96.38% <0%> (ø)	⬆️
openml/datasets/dataset.py	84.07% <0%> (ø)	⬆️
openml/extensions/sklearn/extension.py	91.32% <0%> (+0.11%)	⬆️
openml/datasets/functions.py	95.48% <0%> (+0.32%)	⬆️
... and 1 more

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[amueller]

Not sure what's up with the dict on windows...

[mfeurer]

I think the Windows tests use Python3.5, and therefore, you're dicts are not sorted. Could that be the issue?

[amueller]

Thank you! Sweet :)