update flake8 plugin version and fix warnings (#3180)

.pre-commit-config.yaml

                 .*_pb2_grpc.py
             )$
         # flake8-tidy-imports is used for banned-modules, not actually tidying
-        additional_dependencies: [flake8-comprehensions==3.1.4, flake8-tidy-imports==3.1.0, flake8-bugbear==20.1.2]
+        additional_dependencies: [flake8-comprehensions==3.1.4, flake8-tidy-imports==4.0.0, flake8-bugbear==20.1.2]
     -   id: trailing-whitespace
         name: trailing-whitespace-markdown
         types: [markdown]

gym-unity/gym_unity/tests/test_gym.py

-import unittest.mock as mock
+from unittest import mock
 import pytest
 import numpy as np
 

ml-agents-envs/mlagents_envs/tests/test_envs.py

-import unittest.mock as mock
+from unittest import mock
 import pytest
 
 import numpy as np

ml-agents/mlagents/trainers/tests/mock_brain.py

-import unittest.mock as mock
+from unittest import mock
 import numpy as np
 
 from mlagents.trainers.brain import CameraResolution, BrainParameters

ml-agents/mlagents/trainers/tests/test_agent_processor.py

-import unittest.mock as mock
+from unittest import mock
 import pytest
 import mlagents.trainers.tests.mock_brain as mb
 import numpy as np

ml-agents/mlagents/trainers/tests/test_bcmodule.py

-import unittest.mock as mock
+from unittest import mock
 import pytest
 import mlagents.trainers.tests.mock_brain as mb
 

ml-agents/mlagents/trainers/tests/test_multigpu.py

-import unittest.mock as mock
+from unittest import mock
 import pytest
 
 from mlagents.tf_utils import tf

ml-agents/mlagents/trainers/tests/test_ppo.py

-import unittest.mock as mock
+from unittest import mock
 import pytest
 
 import numpy as np

ml-agents/mlagents/trainers/tests/test_reward_signals.py

-import unittest.mock as mock
+from unittest import mock
 import pytest
 import yaml
 import os

ml-agents/mlagents/trainers/tests/test_sac.py

-import unittest.mock as mock
+from unittest import mock
 import pytest
 import yaml
 

ml-agents/mlagents/trainers/tests/test_stats.py

-import unittest.mock as mock
+from unittest import mock
 import os
 import pytest
 import tempfile

ml-agents/mlagents/trainers/tests/test_subprocess_env_manager.py

-import unittest.mock as mock
+from unittest import mock
 from unittest.mock import Mock, MagicMock
 import unittest
 from queue import Empty as EmptyQueue

ml-agents/mlagents/trainers/tests/test_trainer_util.py

 import io
 from unittest.mock import patch
 
-import mlagents.trainers.trainer_util as trainer_util
+from mlagents.trainers import trainer_util
 from mlagents.trainers.trainer_util import load_config, _load_config
 from mlagents.trainers.ppo.trainer import PPOTrainer
 from mlagents.trainers.exception import TrainerConfigError