Cleanup after remove Python 2 support (#882)

Author: shadchin

tests/config/test_application.py

@@ -14,7 +14,7 @@
 import typing as t
 from io import StringIO
 from tempfile import TemporaryDirectory
-from unittest import TestCase
+from unittest import TestCase, mock
 
 import pytest
 from pytest import mark
@@ -25,11 +25,6 @@
 from traitlets.config.loader import Config, KVArgParseConfigLoader
 from traitlets.tests.utils import check_help_all_output, check_help_output, get_output_error_code
 
-try:
-    from unittest import mock
-except ImportError:
-    from unittest import mock
-
 pjoin = os.path.join