Adding sklearn min. dependencies for all versions

[Neeratyoy]

Reference Issue

Addresses #194.

What does this PR implement/fix? Explain your changes.

Allows exact minimum dependency of packages to be listed in a sklearn flow created under the dependencies attribute.

How should this PR be tested?

Doesn't break any existing unit tests.

Any other comments?

Need to assess how existing flows with different dependency string is affected.

[codecov-io]

Codecov Report

Merging #1022 (463f8dd) into develop (80ae046) will decrease coverage by 0.20%.
The diff coverage is 80.00%.

@@             Coverage Diff             @@
##           develop    #1022      +/-   ##
===========================================
- Coverage    88.23%   88.02%   -0.21%     
===========================================
  Files           36       36              
  Lines         4571     4577       +6     
===========================================
- Hits          4033     4029       -4     
- Misses         538      548      +10     

Impacted Files	Coverage Δ
openml/extensions/sklearn/extension.py	90.71% <80.00%> (-0.14%)	⬇️
openml/config.py	79.67% <0.00%> (-4.42%)	⬇️
openml/testing.py	83.53% <0.00%> (-1.72%)	⬇️
openml/datasets/functions.py	94.42% <0.00%> (-0.04%)	⬇️
openml/utils.py	91.33% <0.00%> (ø)

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