Update test_mdiarea.py

tests/test_mdiarea.py

@@ -5,7 +5,7 @@ from PyQt5.QtGui import QPainter
 
 
 from grum.mainwin import MainWindow
-import mock
+from unittest import mock
 
 from grum.mdi import MDIArea, MDISubMultiPlot, MDISubPlot
 from grum.mdi.mdisubwin import MDISubWindow
@@ -186,4 +186,4 @@ class TestViewUnit(unittest.TestCase):
         assert active == cosine_sub
         for sub in mdi.subWindowList():
             if sub != active:
-                sub.close.assert_called_once()
+                sub.close.assert_called_once()