Use ==/!= to compare str, bytes, and int literals

Created by: cclauss

Identity is not the same thing as equality in Python.

$ python

>>> mode = "tes"
>>> mode += "t"
>>> mode == "test"
True
>>> mode is "test"
False

flake8 testing of https://github.com/deepmipt/DeepPavlov on Python 3.7.1

$ flake8 . --count --select=E9,F63,F7,F82 --show-source --statistics

./deeppavlov/dataset_readers/ubuntu_dstc7_mt_reader.py:92:20: F632 use ==/!= to compare str, bytes, and int literals
                if mode is not "test":
                   ^
./deeppavlov/dataset_readers/ubuntu_dstc7_mt_reader.py:97:20: F632 use ==/!= to compare str, bytes, and int literals
                if mode is not "test":
                   ^
2     F632 use ==/!= to compare str, bytes, and int literals
2