MNT remove deprecation warning in keras example

[glemaitre]

Remove the deprecation warning in keras

[pep8speaks]

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

• In the file examples/applications/porto_seguro_keras_under_sampling.py:

Line 27:1: E402 module level import not at top of file
Line 28:1: E402 module level import not at top of file
Line 29:1: E402 module level import not at top of file
Line 51:1: E402 module level import not at top of file
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Line 86:1: E402 module level import not at top of file
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Line 132:1: E402 module level import not at top of file
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Line 225:1: E402 module level import not at top of file