setup.py: Include 'flake8' in 'test' requirements

We are using this in our testing and actually need it for the flake8
plugin, so include this in the list of 'test' requirements.

Signed-off-by: Stephen Finucane <stephen@that.guru>

tox.ini

@@ -38,7 +38,6 @@ deps=
     {[testenv]deps}
 
 [testenv:flake8]
-deps=flake8
 commands=flake8
 
 [testenv:pylint]