[tox] Put development dependencies into a dev-requirements.txt file
The latter being sourced in tox.ini.
For now this contains references to hg repositories for some dependencies but
under normal circumstances it should only contain pytest.
This is useful when developping cubicweb itself without using tox but working
with a virtualenv so that one can simply do:
::
$ virtualenv --system-site-packages venv
$ . venv/bin/activate
$ pip install -r dev-requirements.txt
$ python setup.py develop