fix to make tests run with 'python setup.py test'

[EdwardBetts]

This is the result of "python setup.py test" before applying this fix.

$ python setup.py test
running test
Traceback (most recent call last):
  File "setup.py", line 57, in <module>
    'Topic :: Utilities',
  File "/usr/lib/python2.7/distutils/core.py", line 151, in setup
    dist.run_commands()
  File "/usr/lib/python2.7/distutils/dist.py", line 953, in run_commands
    self.run_command(cmd)
  File "/usr/lib/python2.7/distutils/dist.py", line 972, in run_command
    cmd_obj.run()
  File "/usr/lib/python2.7/dist-packages/setuptools/command/test.py", line 141, in run
    cmd = ' '.join(self._argv)
TypeError: sequence item 2: expected string, bool found
$ 

[EdwardBetts]

This change doesn't work with python 3.5