Prefer and use built-in unittest.mock in Python 3.3+ instead

of unnecessarily requiring the external mock package.  This helps

distributions that are phasing out Python 2 to remove redundant

packages.

--- a/dev-requirements.txt

+++ b/dev-requirements.txt

@@ -4,7 +4,7 @@ invocations==2.6.0

 pytest==4.4.2

 # pytest-xdist for test dir watching and the inv guard task

 pytest-xdist==1.28.0

-mock==2.0.0

+mock==2.0.0;python_version<"3.3"

 # Linting!

 flake8==3.8.3

 # Formatting!

--- a/tests/test_channelfile.py

+++ b/tests/test_channelfile.py

@@ -1,4 +1,7 @@

-from mock import patch, MagicMock

+try:

+    from unittest.mock import patch, MagicMock

+except ImportError:

+    from mock import patch, MagicMock

 from paramiko import Channel, ChannelFile, ChannelStderrFile, ChannelStdinFile

--- a/tests/test_client.py

+++ b/tests/test_client.py

@@ -34,7 +34,10 @@ import weakref

 from tempfile import mkstemp

 import pytest

-from mock import patch, Mock

+try:

+    from unittest.mock import patch, Mock

+except ImportError:

+    from mock import patch, Mock

 import paramiko

 from paramiko import SSHClient

--- a/tests/test_config.py

+++ b/tests/test_config.py

@@ -11,7 +11,11 @@ try:

 except ImportError:

     Result = None

-from mock import patch

+try:

+    from unittest.mock import patch

+except ImportError:

+    from mock import patch

+

 from pytest import raises, mark, fixture

 from paramiko import (

--- a/tests/test_kex.py

+++ b/tests/test_kex.py

@@ -24,7 +24,11 @@ from binascii import hexlify, unhexlify

 import os

 import unittest

-from mock import Mock, patch

+try:

+    from unittest.mock import Mock, patch

+except ImportError:

+    from mock import Mock, patch

+

 import pytest

 from cryptography.hazmat.backends import default_backend

--- a/tests/test_pkey.py

+++ b/tests/test_pkey.py

@@ -41,7 +41,12 @@ from paramiko.common import o600

 from cryptography.exceptions import UnsupportedAlgorithm

 from cryptography.hazmat.primitives.asymmetric.rsa import RSAPrivateNumbers

-from mock import patch, Mock

+

+try:

+    from unittest.mock import patch, Mock

+except ImportError:

+    from mock import patch, Mock

+

 import pytest

 from .util import _support, is_low_entropy, requires_sha1_signing

--- a/tests/test_proxy.py

+++ b/tests/test_proxy.py

@@ -1,7 +1,11 @@

 import signal

 import socket

-from mock import patch

+try:

+    from unittest.mock import patch

+except ImportError:

+    from mock import patch

+

 from pytest import raises

 from paramiko import ProxyCommand, ProxyCommandFailure

--- a/tests/test_transport.py

+++ b/tests/test_transport.py

@@ -30,7 +30,11 @@ import time

 import threading

 import random

 import unittest

-from mock import Mock

+

+try:

+    from unittest.mock import Mock

+except ImportError:

+    from mock import Mock

 from paramiko import (

     AuthHandler,