Remove dependency on the external module mock

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Since Python 3.3 mock is present the Python’s standard library, so it is not necessary to use external module. This patch removes the need of using it:
```diff
--- a/tests/unit/test_venv.py
+++ b/tests/unit/test_venv.py
@@ -1,4 +1,4 @@
-import mock
+from unittest.mock import patch

import pytest_virtualenv as venv
from pytest_shutil import env
@@ -6,7 +6,7 @@ from pytest_shutil import env

def test_PYTHONPATH_not_present_in_testing_env_if_set():
with env.set_env('PYTHONPATH', 'fred'):
- with mock.patch.object(venv.Workspace, 'run') as run:
+ with patch.object(venv.Workspace, 'run') as run:
venv.VirtualEnv()
call = run.mock_calls[0]
assert 'PYTHONPATH' not in call[2]['env']
@@ -18,7 +18,7 @@ def test_PYTHONPATH_not_present_in_testi

def test_PYTHONPATH_not_present_in_testing_env_if_unset():
with env.no_env('PYTHONPATH'):
- with mock.patch.object(venv.Workspace, 'run') as run:
+ with patch.object(venv.Workspace, 'run') as run:
venv.VirtualEnv()
call = run.mock_calls[0]
assert 'PYTHONPATH' not in call[2]['env']
```

Start with tests/unit/test_venv.py and review its use of the external mock module in the two PYTHONPATH tests. Confirm the standard-library unittest.mock import works with the supported Python version, then run the relevant unit tests; done means the tests pass without importing mock.

Written by the indexing model from the issue text.