Fix F401 flake8 warning (x152 / 268).

This change is mostly autogenerated with:

    $ python -m autoflake --in-place --recursive examples templates transformers utils hubconf.py setup.py

I made minor changes in the generated diff.

templates/adding_a_new_model/tests/modeling_tf_xxx_test.py

@@ -14,7 +14,6 @@
 # limitations under the License.
 from __future__ import absolute_import, division, print_function
 
-import sys
 import unittest
 
 from transformers import XxxConfig, is_tf_available