Remove pytest-runner dependency from setup.py

This dependency should be removed

1. It is deprecated (see https://pypi.org/project/pytest-runner/)
2. We already adopted tox and use it in travis-ci
3. It cause some issues when trying to install source package from local pypi mirror (pytest-runner should be installed separately before pyignite). However, when installing directly from pypi.org, all works ok.

But, unfortunatelly, our TC agents doesn't have tox installed