MNT/CI drop python 3.5

[glemaitre]

Fixing the CI since scikit-learn dropped support for python 3.5

[codecov]

Codecov Report

Merging #683 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #683   +/-   ##
=======================================
  Coverage   98.65%   98.65%           
=======================================
  Files          82       82           
  Lines        4907     4907           
=======================================
  Hits         4841     4841           
  Misses         66       66

Impacted Files	Coverage Δ
imblearn/keras/tests/test_generator.py	100% <0%> (ø)	⬆️
imblearn/tensorflow/_generator.py	100% <0%> (ø)	⬆️
imblearn/keras/_generator.py	98.5% <0%> (ø)	⬆️

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