tests: Try to use unittest.mock from the Python standard library

Only fall back to the standalone mock module if the Python version is
too old to have it in the standard library.

Signed-off-by: Simon McVittie <smcv@debian.org>

dev_requirements.txt

@@ -1,4 +1,4 @@
-mock
+mock; python_version < '3.3'
 
 coverage>=5.0; python_version == '2.7' or python_version >= '3.5'
 pytest-cov>=2.7.0; python_version == '2.7' or python_version >= '3.5'

tests/test_vdf.py

@@ -1,7 +1,11 @@
 import unittest
-import mock
 import sys
 
+try:
+    from unittest import mock
+except ImportError:
+    import mock
+
 try:
         from StringIO import StringIO
 except ImportError: