Declare explicit the directory for pytest

[Henri Hulski]

I just try to integrate travis and tox with morepath_cerebral_todomvc.
Now I have the problem, that pytest scans the whole .tox directory for tests, which contains also the site-packages.

So I think it's the best not providing a blacklist to just declare the path to check for pythest in setup.cfg:

[pytest]
testpaths = {{cookiecutter.package_name.split('.')[0]}}