address Nyall's comments:

- add TransferThis to parent argument
 - QSignalSpy always available with Qt5

python/gui/qgscheckablecombobox.sip

@@ -23,7 +23,7 @@ class QgsCheckableComboBox : QComboBox
 
   public:
 
-    QgsCheckableComboBox( QWidget *parent = 0 );
+    QgsCheckableComboBox( QWidget *parent /TransferThis/ = 0 );
 %Docstring
  Constructor for QgsCheckableComboBox.
  \param parent parent object

src/gui/qgscheckablecombobox.h

@@ -124,7 +124,7 @@ class GUI_EXPORT QgsCheckableComboBox : public QComboBox
     /** Constructor for QgsCheckableComboBox.
      * \param parent parent object
      */
-    QgsCheckableComboBox( QWidget *parent = nullptr );
+    QgsCheckableComboBox( QWidget *parent SIP_TRANSFERTHIS = nullptr );
 
     /** Returns separator used to separate items in the display text.
      * \see setSeparator()

tests/src/python/test_qgscheckablecombobox.py

@@ -15,14 +15,9 @@ __revision__ = '$Format:%H$'
 import qgis  # NOQA
 
 from qgis.PyQt.QtCore import Qt
+from qgis.PyQt.QtTest import QSignalSpy
+
 from qgis.gui import QgsCheckableComboBox
-
-try:
-    from qgis.PyQt.QtTest import QSignalSpy
-    use_signal_spy = True
-except:
-    use_signal_spy = False
-
 from qgis.testing import start_app, unittest
 
 start_app()
@@ -51,7 +46,6 @@ class TestQgsCheckableComboBox(unittest.TestCase):
         w.setItemCheckState(2, Qt.Unchecked)
         self.assertEqual(w.itemCheckState(2), Qt.Unchecked)
 
-    @unittest.skipIf(not use_signal_spy, "No QSignalSpy available")
     def test_ChangedSignals(self):
         """ test that signals are correctly emitted when clearing"""