Prefer unittest.mock over external mock package

Prefer using unittest.mock over external mock package to avoid
extraneous dependencies in Python 3.3+.  Install 'mock' only for older
Python versions.

Author: mgorny

setup.py

@@ -63,6 +63,6 @@ def readall(path):
         "Programming Language :: Python :: Implementation :: PyPy",
     ],
     zip_safe=False,
-    tests_require=["mock", "requests-mock"],
+    tests_require=['mock;python_version<"3.3"', "requests-mock"],
     test_suite="tests",
 )

tests/test_core.py

@@ -8,9 +8,9 @@
 import unittest
 
 try:
-    import mock
-except ImportError:
     from unittest import mock
+except ImportError:
+    import mock
 
 
 @mock.patch("oauthlib.oauth1.rfc5849.generate_timestamp")

tests/test_oauth1_session.py

@@ -9,9 +9,9 @@
 from requests_oauthlib import OAuth1Session
 
 try:
-    import mock
-except ImportError:
     from unittest import mock
+except ImportError:
+    import mock
 
 try:
     import cryptography

tests/test_oauth2_session.py

@@ -9,9 +9,9 @@
 from unittest import TestCase
 
 try:
-    import mock
-except ImportError:
     from unittest import mock
+except ImportError:
+    import mock
 
 from oauthlib.common import urlencode
 from oauthlib.oauth2 import TokenExpiredError, OAuth2Error

tox.ini

@@ -4,7 +4,7 @@ envlist = py27, py34, py35, py36, py37, pypy, pypy3
 [testenv]
 deps=
     -r{toxinidir}/requirements.txt
-    mock
+    mock;python_version<"3.3"
     coveralls
     requests-mock
 commands= coverage run --source=requests_oauthlib -m unittest discover