Update test_mainwin.py

tests/test_mainwin.py

@@ -5,7 +5,7 @@ import sys
 from PyQt5.QtWidgets import QApplication, QAction, QListWidget
 from grum.mainwin import MainWindow
 from grum.dictlist import DictList
-import mock
+from unittest import mock
 from grum import ctrl_c, theme, assets
 
 from grum.dictlist.dictlistitem import DictListItem