Bug report: AttributeError: type object 'QFont' has no attribute 'Decorative'

## Environment

OS: Linux
Python version: 3.11
Toolkit: Qt
Qt API: PyQt6

pyface/traitsui 8.0.0

## Description

When running the traitsui pyqt6 tests I get:

```
ERROR: test_create_traitsfont_pyface_font (traitsui.qt.tests.test_font_trait.TestPyQtFont.test_create_traitsfont_pyface_font)
----------------------------------------------------------------------
Traceback (most recent call last):
File "/builddir/build/BUILDROOT/python-traitsui-8.0.0-1.fc39.x86_64/usr/lib/python3.11/site-packages/traitsui/qt/tests/test_font_trait.py", line 89, in test_create_traitsfont_pyface_font
traits_font = create_traitsfont(font)
^^^^^^^^^^^^^^^^^^^^^^^
File "/builddir/build/BUILDROOT/python-traitsui-8.0.0-1.fc39.x86_64/usr/lib/python3.11/site-packages/traitsui/qt/font_trait.py", line 94, in create_traitsfont
return TraitsFont(value.to_toolkit())
^^^^^^^^^^^^^^^^^^
File "/usr/lib/python3.11/site-packages/pyface/font.py", line 378, in to_toolkit
font_to_toolkit_font = toolkit_object('font:font_to_toolkit_font')
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/usr/lib/python3.11/site-packages/pyface/base_toolkit.py", line 127, in __call__
module = import_module(mname, package)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/usr/lib64/python3.11/importlib/__init__.py", line 126, in import_module
return _bootstrap._gcd_import(name[level:], package, level)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "", line 1206, in _gcd_import
File "", line 1178, in _find_and_load
File "", line 1149, in _find_and_load_unlocked
File "", line 690, in _load_unlocked
File "", line 940, in exec_module
File "", line 241, in _call_with_frames_removed
File "/usr/lib/python3.11/site-packages/pyface/ui/qt/font.py", line 25, in
QFont.Decorative: 'fantasy',
^^^^^^^^^^^^^^^^
AttributeError: type object 'QFont' has no attribute 'Decorative'
```
With PyQt6 this is now QFont.StyleHint.Decorative. I don't know how pyface is supposed to handle this difference.