WxPython 4.1 clipboard test failures

The following test failures occur on MacOS under WxPython 4.1, but not under WxPython 4.0:
```

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FAIL: test_set_object_data (pyface.tests.test_clipboard.TestClipboard)
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Traceback (most recent call last):
File "/Users/cwebster/.edm/envs/pyface-test-3.6-wx/lib/python3.6/site-packages/pyface-7.1.0.dev7-py3.6.egg/pyface/tests/test_clipboard.py", line 67, in test_set_object_data
self.clipboard.data = data
File "/Users/cwebster/.edm/envs/pyface-test-3.6-wx/lib/python3.6/site-packages/pyface-7.1.0.dev7-py3.6.egg/pyface/i_clipboard.py", line 111, in _set_data
self.object_data = data
File "/Users/cwebster/.edm/envs/pyface-test-3.6-wx/lib/python3.6/site-packages/pyface-7.1.0.dev7-py3.6.egg/pyface/ui/wx/clipboard.py", line 78, in _set_object_data
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)
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Traceback (most recent call last):
File "/Users/cwebster/.edm/envs/pyface-test-3.6-wx/lib/python3.6/site-packages/pyface-7.1.0.dev7-py3.6.egg/pyface/tests/test_clipboard.py", line 35, in test_set_text_data
self.clipboard.data = "test"
File "/Users/cwebster/.edm/envs/pyface-test-3.6-wx/lib/python3.6/site-packages/pyface-7.1.0.dev7-py3.6.egg/pyface/i_clipboard.py", line 107, in _set_data
self.text_data = data
File "/Users/cwebster/.edm/envs/pyface-test-3.6-wx/lib/python3.6/site-packages/pyface-7.1.0.dev7-py3.6.egg/pyface/ui/wx/clipboard.py", line 117, in _set_text_data
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)
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Traceback (most recent call last):
File "/Users/cwebster/.edm/envs/pyface-test-3.6-wx/lib/python3.6/site-packages/pyface-7.1.0.dev7-py3.6.egg/pyface/tests/test_clipboard.py", line 45, in test_set_text_data_unicode
self.clipboard.data = u"test"
File "/Users/cwebster/.edm/envs/pyface-test-3.6-wx/lib/python3.6/site-packages/pyface-7.1.0.dev7-py3.6.egg/pyface/i_clipboard.py", line 107, in _set_data
self.text_data = data
File "/Users/cwebster/.edm/envs/pyface-test-3.6-wx/lib/python3.6/site-packages/pyface-7.1.0.dev7-py3.6.egg/pyface/ui/wx/clipboard.py", line 117, in _set_text_data
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

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```