Remove test for default-constructed wxPGWindowList, use Getters

The default constructor has been removed in wxWidgets 3.1.4, commit
https://github.com/wxWidgets/wxWidgets/commit/f2f9cbe61960
"(Get rid of unused wxPGWindowList ctor").

Now, at least the primary is required, the secondary is defaulted.

The members are no longer public in the interface.

unittests/test_propgrideditors.py

@@ -9,12 +9,13 @@ class propgrideditors_Tests(wtc.WidgetTestCase):
 
 
     def test_propgrideditors01(self):
-        wl = pg.PGWindowList()
-        assert wl.m_primary is None
-        assert wl.m_secondary is None
-
         wl = pg.PGWindowList(self.frame)
-        assert wl.m_primary is self.frame
+        assert wl.GetPrimary() is self.frame
+        assert wl.GetSecondary() is None
+
+        wl = pg.PGWindowList(self.frame, None)
+        assert wl.GetPrimary() is self.frame
+        assert wl.GetSecondary() is None
 
 
     def test_propgrideditors02(self):