ENH make lazy import of the keras module when importing imblearn

[glemaitre]

closes #704

Lazy import of keras when importing imblearn.

[codecov]

Codecov Report

Merging #719 into master will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #719   +/-   ##
=======================================
  Coverage   96.47%   96.47%           
=======================================
  Files          82       82           
  Lines        5026     5026           
=======================================
  Hits         4849     4849           
  Misses        177      177           

Impacted Files	Coverage Δ
imblearn/utils/_validation.py	100.00% <ø> (ø)
imblearn/ensemble/_bagging.py	97.72% <100.00%> (ø)
imblearn/ensemble/_easy_ensemble.py	100.00% <100.00%> (ø)
imblearn/ensemble/_forest.py	97.33% <100.00%> (ø)
imblearn/utils/estimator_checks.py	95.60% <100.00%> (ø)

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