Retain compatibility with Spyder 6.0.0 alpha 2

That is the latest version currently on conda-forge, so half of the GitHub
automatic tests use Spyder 6.0.0 alpha 2.

spyder_unittest/widgets/tests/test_confpage.py

@@ -127,7 +127,12 @@ def config_dialog(qtbot, request):
     qtbot.addWidget(main_ref)
 
     preferences = main_ref._main.get_plugin(Plugins.Preferences)
-    preferences.open_dialog()
+    try:
+        # Spyder 6.0.0 alpha 3 and higher
+        preferences.open_dialog()
+    except TypeError:
+        # Spyder 6.0.0 alpha 2 and lower
+        preferences.open_dialog(None)
     container = preferences.get_container()
     dlg = container.dialog