Remove a test that uses UITester (#1788)

This PR removes a test that's using `UITester` from TraitsUI. This test
exposed the Traits test suite to a class of possible issues with 3rd
party GUI frameworks (in this case PySide6); historically, `UITester`
has also been a particularly fragile part of TraitsUI.

In its place is a test that at least executes the `Enum.create_editor`
method, but doesn't attempt to create an instance of the editor.

We also fix an issue with test dependencies and the latest release of
Sphinx: `defusedxml` is now required to run the tests for the Traits
documenter.

Closes #1787

setup.py

@@ -302,6 +302,9 @@ def get_long_description():
         ],
         "test": [
             "Cython",
+            # defusedxml is required by the Sphinx test machinery
+            # for recent versions of Sphinx (including 7.3.7)
+            "defusedxml",
             "flake8",
             "flake8-ets",
             "mypy",

traits/tests/test_enum.py

@@ -14,11 +14,8 @@
 from traits.api import (
     Any, BaseEnum, Enum, HasTraits, Int, List, Property, Set, TraitError,
     Tuple)
-from traits.etsconfig.api import ETSConfig
 from traits.testing.optional_dependencies import requires_traitsui
 
-is_null = ETSConfig.toolkit == 'null'
-
 
 class FooEnum(enum.Enum):
     foo = 0
@@ -115,13 +112,6 @@ class EnumCollectionExample(HasTraits):
     slow_enum = BaseEnum("yes", "no", "maybe")
 
 
-class EnumCollectionGUIExample(EnumCollectionExample):
-    # Override attributes that may fail GUI test
-    # until traitsui #781 is fixed.
-    int_set_enum = Enum("int", "set")
-    correct_int_set_enum = Enum("int", "set")
-
-
 class EnumTestCase(unittest.TestCase):
     def test_valid_enum(self):
         example_model = ExampleModel(root="model1")
@@ -343,12 +333,14 @@ class HasEnumInList(HasTraits):
 
 
 @requires_traitsui
-@unittest.skipIf(is_null, "GUI toolkit not available")
-class TestGui(unittest.TestCase):
-
+class TestEnumCreateEditor(unittest.TestCase):
     def test_create_editor(self):
-        from traitsui.testing.api import UITester
-
-        obj = EnumCollectionGUIExample()
-        with UITester().create_ui(obj):
-            pass
+        import traitsui.editor_factory
+
+        obj = EnumListExample()
+        trait = obj.trait("value")
+        editor_factory = trait.trait_type.create_editor()
+        self.assertIsInstance(
+            editor_factory,
+            traitsui.editor_factory.EditorFactory
+        )