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

Author: chfw

tests/test_repo.py

@@ -1,6 +1,5 @@
 import fs.path
 import fs.errors
-from mock import MagicMock, patch
 from nose.tools import eq_, raises
 from gitfs2.repo import (
     GitRequire,
@@ -10,6 +9,11 @@
     make_sure_git_is_available,
 )
 
+try:
+    from mock import MagicMock, patch
+except ImportError:
+    from unittest.mock import MagicMock, patch
+
 
 @patch("appdirs.user_cache_dir", return_value="root")
 @patch("gitfs2.repo.mkdir_p")