Errors occur in test_clipboard.py on latest wx

When running the testsuite with wxpython 4.1.1 the following errors occur:
```
======================================================================
FAIL: test_set_object_data (pyface.tests.test_clipboard.TestClipboard)
----------------------------------------------------------------------
Traceback (most recent call last):
File "/Users/aayres/Desktop/pyface/pyface/tests/test_clipboard.py", line 71, in test_set_object_data
self.clipboard.data = data
File "/Users/aayres/Desktop/pyface/pyface/i_clipboard.py", line 108, in _set_data
self.object_data = data
File "/Users/aayres/Desktop/pyface/pyface/ui/wx/clipboard.py", line 125, in _set_object_data
cb.SetData(cdo)
File "/Users/aayres/.edm/envs/test-pyface-7.2.0-wx/lib/python3.6/contextlib.py", line 88, in __exit__
next(self.gen)
File "/Users/aayres/Desktop/pyface/pyface/ui/wx/clipboard.py", line 70, in _close_clipboard
cb.Flush()
wx._core.wxAssertionError: C++ assertion ""m_open"" failed at /Users/robind/projects/bb2/dist-osx-py36/build/ext/wxWidgets/src/osx/carbon/clipbrd.cpp(64) in Flush(): clipboard not open

======================================================================
FAIL: test_set_text_data (pyface.tests.test_clipboard.TestClipboard)
----------------------------------------------------------------------
Traceback (most recent call last):
File "/Users/aayres/Desktop/pyface/pyface/tests/test_clipboard.py", line 39, in test_set_text_data
self.clipboard.data = "test"
File "/Users/aayres/Desktop/pyface/pyface/i_clipboard.py", line 104, in _set_data
self.text_data = data
File "/Users/aayres/Desktop/pyface/pyface/ui/wx/clipboard.py", line 164, in _set_text_data
cb.SetData(wx.TextDataObject(str(data)))
File "/Users/aayres/.edm/envs/test-pyface-7.2.0-wx/lib/python3.6/contextlib.py", line 88, in __exit__
next(self.gen)
File "/Users/aayres/Desktop/pyface/pyface/ui/wx/clipboard.py", line 70, in _close_clipboard
cb.Flush()
wx._core.wxAssertionError: C++ assertion ""m_open"" failed at /Users/robind/projects/bb2/dist-osx-py36/build/ext/wxWidgets/src/osx/carbon/clipbrd.cpp(64) in Flush(): clipboard not open

======================================================================
FAIL: test_set_text_data_unicode (pyface.tests.test_clipboard.TestClipboard)
----------------------------------------------------------------------
Traceback (most recent call last):
File "/Users/aayres/Desktop/pyface/pyface/tests/test_clipboard.py", line 49, in test_set_text_data_unicode
self.clipboard.data = "test"
File "/Users/aayres/Desktop/pyface/pyface/i_clipboard.py", line 104, in _set_data
self.text_data = data
File "/Users/aayres/Desktop/pyface/pyface/ui/wx/clipboard.py", line 164, in _set_text_data
cb.SetData(wx.TextDataObject(str(data)))
File "/Users/aayres/.edm/envs/test-pyface-7.2.0-wx/lib/python3.6/contextlib.py", line 88, in __exit__
next(self.gen)
File "/Users/aayres/Desktop/pyface/pyface/ui/wx/clipboard.py", line 70, in _close_clipboard
cb.Flush()
wx._core.wxAssertionError: C++ assertion ""m_open"" failed at /Users/robind/projects/bb2/dist-osx-py36/build/ext/wxWidgets/src/osx/carbon/clipbrd.cpp(64) in Flush(): clipboard not open

---------------------------------------------------------------------
```

relevant code:
https://github.com/enthought/pyface/blob/92ab7363268ddcf9efde8fc4cdc561b042160e90/pyface/ui/wx/clipboard.py#L55-L70
https://github.com/wxWidgets/wxWidgets/blob/3ec166f960b0251f01d6b4886c4309327f7af366/src/osx/carbon/clipbrd.cpp#L64