Chaco & WX fail

Hello,

Chaco fails to run with ETS_TOOLKIT set to "wx"

For instance, any example in Chaco demo directory fail :

```
python3 scatter.py

Traceback (most recent call last):
File "scatter.py", line 22, in
from enable.api import Component, ComponentEditor
File "/usr/local/lib/python3.7/dist-packages/enable-4.8.1.dev52-py3.7-linux-x86_64.egg/enable/api.py", line 21, in
from .markers import MarkerTrait, marker_trait, MarkerNameDict, marker_names, \
File "/usr/local/lib/python3.7/dist-packages/enable-4.8.1.dev52-py3.7-linux-x86_64.egg/enable/markers.py", line 18, in
from .compiled_path import CompiledPath
File "/usr/local/lib/python3.7/dist-packages/enable-4.8.1.dev52-py3.7-linux-x86_64.egg/enable/compiled_path.py", line 17, in
from .toolkit import toolkit_object
File "/usr/local/lib/python3.7/dist-packages/enable-4.8.1.dev52-py3.7-linux-x86_64.egg/enable/toolkit.py", line 47, in
_init_toolkit()
File "/usr/local/lib/python3.7/dist-packages/enable-4.8.1.dev52-py3.7-linux-x86_64.egg/enable/toolkit.py", line 35, in _init_toolkit
__import__(backend)
File "/usr/local/lib/python3.7/dist-packages/enable-4.8.1.dev52-py3.7-linux-x86_64.egg/enable/wx/image.py", line 18, in
from .base_window import BaseWindow
File "/usr/local/lib/python3.7/dist-packages/enable-4.8.1.dev52-py3.7-linux-x86_64.egg/enable/wx/base_window.py", line 22, in
from .constants import (
File "/usr/local/lib/python3.7/dist-packages/enable-4.8.1.dev52-py3.7-linux-x86_64.egg/enable/wx/constants.py", line 127, in
wx.WXK_NEXT,
AttributeError: module 'wx' has no attribute 'WXK_NEXT'
```

Debian 10 x86_64 (stable), Python 3.7.3, wx 4.0.4 gtk3 (phoenix) wxWidgets 3.0.4 with ETS source code updated

Regards