drop Python2 support and old Mock requirement on Py3

tests/test_outcome_response.py

@@ -1,6 +1,6 @@
 from lti import OutcomeResponse
 from lxml import etree
-import mock
+from unittest import mock
 import unittest
 
 RESPONSE_XML = b"""<?xml version="1.0" encoding="UTF-8"?>

tests/test_tool_provider.py

@@ -1,11 +1,7 @@
 import unittest
+from unittest.mock import Mock, patch
+from urllib.parse import urlsplit
 
-try:
-    from urllib.parse import urlsplit
-except ImportError:
-    from urlparse import urlsplit  # Python 2
-
-from mock import Mock, patch
 from oauthlib.common import generate_client_id
 from oauthlib.common import generate_token
 from oauthlib.oauth1 import SignatureOnlyEndpoint

tests/test_tool_proxy.py

@@ -1,5 +1,6 @@
 import unittest
-from mock import Mock, patch
+from unittest.mock import Mock, patch
+
 from lti import ToolProxy
 import requests
 import json
@@ -213,4 +214,4 @@ def test_register_proxy(self):
 
         signed_request = proxy.register_proxy({'tool_profile': 'A Real Tool Profile Goes here'})
 
-        self.assertIsInstance(signed_request, requests.PreparedRequest)
+        self.assertIsInstance(signed_request, requests.PreparedRequest)

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-envlist = py27, py35, py36, py37, py38
+envlist = py35, py36, py37, py38
 
 [testenv]
 commands =