Renamed singleShot to isSingleShot in QTimer stub

As in compliance to: http://doc.qt.io/qt-5/qtimer.html

test/test_stubs.py

@@ -82,9 +82,9 @@ def test_disconnect_one_invalid(timer):
 
 def test_singleshot(timer):
     """Test setting singleShot."""
-    assert not timer.singleShot()
+    assert not timer.isSingleShot()
     timer.setSingleShot(True)
-    assert timer.singleShot()
+    assert timer.isSingleShot()
     timer.start()
     assert timer.isActive()
     timer.timeout.emit()