FIX: rename sparse to keep_sparse for keras and tensorflow

[glemaitre]

No description provided.

[pep8speaks]

Hello @glemaitre! Thanks for submitting the PR.

• In the file imblearn/keras/tests/test_generator.py, following are the PEP8 issues :

Line 9:1: E402 module level import not at top of file
Line 10:1: E402 module level import not at top of file
Line 11:1: E402 module level import not at top of file
Line 13:1: E402 module level import not at top of file
Line 14:1: E402 module level import not at top of file
Line 15:1: E402 module level import not at top of file
Line 17:1: E402 module level import not at top of file
Line 18:1: E402 module level import not at top of file