💚 fix mock dependency issue. python 3 has it in unittest so we are not installing it in python 3

Author: chfw

tests/test_gitfs_opener.py

@@ -1,5 +1,9 @@
 import fs
-from mock import patch
+try:
+    from mock import patch
+except ImportError:
+    from unittest.mock import patch
+
 from nose.tools import ok_
 from gitfs2.repo import GitRequire