Remove the deprecated class creation listeners (#1730)

This PR removes the deprecated `MetaHasTraits.add_listener` and
`MetaHasTraits.remove_listener` methods.

**Checklist**
- [x] Tests
- [x] Update API reference (`docs/source/traits_api_reference`) - N/A:
feature was not documented
- [x] Update User manual (`docs/source/traits_user_manual`) - N/A:
feature was not documented
- [x] Update type annotation hints in stub files

For the rationale for the deprecation, see #1549.

traits/has_traits.py

@@ -390,54 +390,12 @@ class MetaHasTraits(type):
 
     """
 
-    # All registered class creation listeners.
-    #
-    # { Str class_name : Callable listener }
-    _listeners = {}
-
     def __new__(cls, class_name, bases, class_dict):
+        # Convert entries in the class dictionary into traits, as appropriate.
         update_traits_class_dict(class_name, bases, class_dict)
 
-        # Finish building the class using the updated class dictionary:
-        klass = type.__new__(cls, class_name, bases, class_dict)
-
-        # Call all listeners that registered for this specific class:
-        name = "%s.%s" % (klass.__module__, klass.__name__)
-        for listener in MetaHasTraits._listeners.get(name, []):
-            listener(klass)
-
-        # Call all listeners that registered for ANY class:
-        for listener in MetaHasTraits._listeners.get("", []):
-            listener(klass)
-
-        return klass
-
-    @classmethod
-    def add_listener(cls, listener, class_name=""):
-        """ Adds a class creation listener.
-
-        If the class name is the empty string then the listener will be called
-        when *any* class is created.
-
-        .. deprecated:: 6.3.0
-        """
-        warnings.warn(
-            "add_listener is deprecated", DeprecationWarning, stacklevel=2
-        )
-
-        MetaHasTraits._listeners.setdefault(class_name, []).append(listener)
-
-    @classmethod
-    def remove_listener(cls, listener, class_name=""):
-        """ Removes a class creation listener.
-
-        .. deprecated:: 6.3.0
-        """
-        warnings.warn(
-            "remove_listener is deprecated", DeprecationWarning, stacklevel=2
-        )
-
-        MetaHasTraits._listeners[class_name].remove(listener)
+        # Finish building the class using the updated class dictionary.
+        return type.__new__(cls, class_name, bases, class_dict)
 
 
 def update_traits_class_dict(class_name, bases, class_dict):

traits/has_traits.pyi

@@ -49,10 +49,6 @@ class _SimpleTest:
 
 class MetaHasTraits(type):
     def __new__(cls, class_name: _Any, bases: _Any, class_dict: _Any): ...
-    @classmethod
-    def add_listener(cls, listener: _Any, class_name: str = ...) -> None: ...
-    @classmethod
-    def remove_listener(cls, listener: _Any, class_name: str = ...) -> None: ...
 
 def update_traits_class_dict(class_name: _Any, bases: _Any, class_dict: _Any): ...
 def migrate_property(name: _Any, property: _Any, property_info: _Any, class_dict: _Any): ...

traits/tests/test_has_traits.py

@@ -26,7 +26,6 @@
     SingletonHasTraits,
     SingletonHasStrictTraits,
     SingletonHasPrivateTraits,
-    MetaHasTraits,
 )
 from traits.ctrait import CTrait
 from traits.observation.api import (
@@ -56,21 +55,6 @@ def _dummy_validator(self, value):
     pass
 
 
-class TestMetaHasTraits(unittest.TestCase):
-    def test_add_listener_and_remove_listener_deprecated(self):
-        def listener(cls):
-            pass
-
-        with self.assertWarnsRegex(
-            DeprecationWarning, "add_listener is deprecated"
-        ):
-            MetaHasTraits.add_listener(listener)
-        with self.assertWarnsRegex(
-            DeprecationWarning, "remove_listener is deprecated"
-        ):
-            MetaHasTraits.remove_listener(listener)
-
-
 class TestCreateTraitsMetaDict(unittest.TestCase):
     def test_class_attributes(self):
         # Given