[MRG] MAINT drop python 2 and upgrade scikit-learn 0.21

[glemaitre]

closes #574
closes #570

[pep8speaks]

Hello @glemaitre! Thanks for updating this PR. We checked the lines you've touched for PEP 8 issues, and found:

• In the file doc/conf.py:

Line 334:80: E501 line too long (83 > 79 characters)

• In the file doc/sphinxext/sphinx_issues.py:

Line 32:80: E501 line too long (80 > 79 characters)
Line 83:80: E501 line too long (84 > 79 characters)
Line 106:80: E501 line too long (85 > 79 characters)
Line 109:80: E501 line too long (82 > 79 characters)

Comment last updated at 2019-06-07 18:55:59 UTC

[codecov]

Codecov Report

Merging #576 into master will decrease coverage by 0.03%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #576      +/-   ##
==========================================
- Coverage   98.89%   98.85%   -0.04%     
==========================================
  Files          84       84              
  Lines        5154     5151       -3     
==========================================
- Hits         5097     5092       -5     
- Misses         57       59       +2

Impacted Files	Coverage Δ
imblearn/utils/tests/test_docstring.py	76.92% <ø> (ø)	⬆️
imblearn/utils/testing.py	100% <ø> (ø)	⬆️
...rototype_selection/_instance_hardness_threshold.py	100% <100%> (ø)	⬆️
...ling/_prototype_selection/_random_under_sampler.py	100% <100%> (ø)	⬆️
imblearn/combine/_smote_tomek.py	100% <100%> (ø)	⬆️
imblearn/ensemble/_easy_ensemble.py	100% <100%> (ø)	⬆️
...rototype_selection/_neighbourhood_cleaning_rule.py	100% <100%> (ø)	⬆️
imblearn/utils/_docstring.py	100% <100%> (ø)	⬆️
...nder_sampling/_prototype_selection/_tomek_links.py	100% <100%> (ø)	⬆️
imblearn/utils/tests/test_deprecation.py	100% <100%> (ø)	⬆️
... and 30 more

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