Test failure in clipboard code on wxPython

In #616 we are getting a segfault in the `test_set_object_data` test for the clipboard object.
```
test_set_object_data (pyface.tests.test_clipboard.TestClipboard) ... Fatal Python error: Segmentation fault
Current thread 0x00007fb8d7701700 (most recent call first):
File "/home/travis/.edm/envs/pyface-test-3.6-wx/lib/python3.6/site-packages/pyface-7.1.0.dev61-py3.6.egg/pyface/ui/wx/clipboard.py", line 60 in _get_object_data
File "/home/travis/.edm/envs/pyface-test-3.6-wx/lib/python3.6/site-packages/pyface-7.1.0.dev61-py3.6.egg/pyface/i_clipboard.py", line 102 in _get_data
File "/home/travis/.edm/envs/pyface-test-3.6-wx/lib/python3.6/site-packages/pyface-7.1.0.dev61-py3.6.egg/pyface/tests/test_clipboard.py", line 70 in test_set_object_data
File "/home/travis/.edm/envs/pyface-test-3.6-wx/lib/python3.6/unittest/case.py", line 605 in run
File "/home/travis/.edm/envs/pyface-test-3.6-wx/lib/python3.6/unittest/case.py", line 653 in __call__
File "/home/travis/.edm/envs/pyface-test-3.6-wx/lib/python3.6/unittest/suite.py", line 122 in run
File "/home/travis/.edm/envs/pyface-test-3.6-wx/lib/python3.6/unittest/suite.py", line 84 in __call__
File "/home/travis/.edm/envs/pyface-test-3.6-wx/lib/python3.6/unittest/suite.py", line 122 in run
File "/home/travis/.edm/envs/pyface-test-3.6-wx/lib/python3.6/unittest/suite.py", line 84 in __call__
File "/home/travis/.edm/envs/pyface-test-3.6-wx/lib/python3.6/unittest/suite.py", line 122 in run
File "/home/travis/.edm/envs/pyface-test-3.6-wx/lib/python3.6/unittest/suite.py", line 84 in __call__
File "/home/travis/.edm/envs/pyface-test-3.6-wx/lib/python3.6/unittest/suite.py", line 122 in run
File "/home/travis/.edm/envs/pyface-test-3.6-wx/lib/python3.6/unittest/suite.py", line 84 in __call__
File "/home/travis/.edm/envs/pyface-test-3.6-wx/lib/python3.6/unittest/runner.py", line 176 in run
File "/home/travis/.edm/envs/pyface-test-3.6-wx/lib/python3.6/unittest/main.py", line 256 in runTests
File "/home/travis/.edm/envs/pyface-test-3.6-wx/lib/python3.6/unittest/main.py", line 95 in __init__
File "/home/travis/.edm/envs/pyface-test-3.6-wx/lib/python3.6/unittest/__main__.py", line 18 in
File "/home/travis/.edm/envs/pyface-test-3.6-wx/lib/python3.6/site-packages/coverage/execfile.py", line 183 in run_python_file
File "/home/travis/.edm/envs/pyface-test-3.6-wx/lib/python3.6/site-packages/coverage/execfile.py", line 113 in run_python_module
File "/home/travis/.edm/envs/pyface-test-3.6-wx/lib/python3.6/site-packages/coverage/cmdline.py", line 627 in do_run
File "/home/travis/.edm/envs/pyface-test-3.6-wx/lib/python3.6/site-packages/coverage/cmdline.py", line 483 in command_line
File "/home/travis/.edm/envs/pyface-test-3.6-wx/lib/python3.6/site-packages/coverage/cmdline.py", line 756 in main
File "/home/travis/.edm/envs/pyface-test-3.6-wx/lib/python3.6/site-packages/coverage/__main__.py", line 8 in
File "/home/travis/.edm/envs/pyface-test-3.6-wx/lib/python3.6/runpy.py", line 85 in _run_code
File "/home/travis/.edm/envs/pyface-test-3.6-wx/lib/python3.6/runpy.py", line 193 in _run_module_as_main
Command "python -Xfaulthandler -m coverage run -p -m unittest discover -v pyface" exited with segmentation fault (-11)
Command '['/home/travis/.edm/envs/edm/bin/edm', 'run', '-e', 'pyface-test-3.6-wx', '--', 'python', '-Xfaulthandler', '-m', 'coverage', 'run', '-p', '-m', 'unittest', 'discover', '-v', 'pyface']' returned non-zero exit status 245.
```