Fix pylint warnings

Signed-off-by: Beat Buesser <[email protected]>

Author: Beat Buesser

art/attacks/evasion/pixel_threshold.py

@@ -22,7 +22,7 @@
 | One Pixel Attack Paper link: https://arxiv.org/ans/1710.08864
 | Pixel and Threshold Attack Paper link: https://arxiv.org/abs/1906.06026
 """
-# pylint: disable=C0302,C0413
+# pylint: disable=C0302,C0413,E402
 from __future__ import absolute_import, division, print_function, unicode_literals
 
 import logging