drop dependency on old external mock module (#385)

requirements-test.txt

@@ -4,6 +4,5 @@ isort == 5.13.2
 mypy == 1.10.0
 pytest
 pytest-cov
-mock
 types-requests
-types-xmltodict
+types-xmltodict

winrm/tests/conftest.py

@@ -1,9 +1,9 @@
 # flake8: noqa
 import os
 import uuid
+from unittest.mock import patch
 
 import xmltodict
-from mock import patch
 from pytest import fixture, skip
 
 open_shell_request = """\

winrm/tests/test_transport.py

@@ -1,8 +1,7 @@
 # coding=utf-8
 import os
 import unittest
-
-import mock
+from unittest import mock
 
 from winrm import transport
 from winrm.exceptions import InvalidCredentialsError, WinRMError