Switch to unittest.mock (#78)

The mock package has been integrated into unittest since Python 3.3,
which includes all supported distributions of ROS 2.

Signed-off-by: Scott K Logan <[email protected]>

Author: cottsay

package.xml

@@ -25,7 +25,6 @@
 
   <exec_depend>python3-lxml</exec_depend>
   <exec_depend>python3-yaml</exec_depend>
-  <test_depend>python3-mock</test_depend>
 
   <export>
     <build_type>ament_python</build_type>

test/test_urdf.py

@@ -1,6 +1,6 @@
 
 import unittest
-import mock
+from unittest import mock
 import os
 import sys