TST: do not use deprecated and removed mpl backend name in test

Qt4 support is no longer a thing.

IPython/core/tests/test_pylabtools.py

@@ -173,8 +173,8 b' class TestPylabSwitch(object):'
         # Save rcParams since they get modified
         self._saved_rcParams = matplotlib.rcParams
         self._saved_rcParamsOrig = matplotlib.rcParamsOrig
-        matplotlib.rcParams = dict(backend='Qt4Agg')
-        matplotlib.rcParamsOrig = dict(backend='Qt4Agg')
+        matplotlib.rcParams = dict(backend='QtAgg')
+        matplotlib.rcParamsOrig = dict(backend='QtAgg')
 
         # Mock out functions
         self._save_am = pt.activate_matplotlib