Specify astroid<2.0 for setuptools

[tnir]

Due to missing ignore of pre-release packages in setuptools pypa/setuptools#855 and the existence of astroid pre-released 2.0.0.dev0, we should encounter the following failure when some packages (e.g., pylint) are installed together.

setup.py

setuptools.setup(
    install_requires=[
        'pylint==1.9.1',
        'requirements-detector==0.5.2'
    ],
)

Results with failures

See https://readthedocs.org/projects/prospector/builds/7247785/ (https://github.com/PyCQA/prospector project's docs build).

Solutions

To prevent this problem, we can add a workaround specifying astroid<2.0 to this requirements-detector project's requirements in its setup.py.

[carlio]

No longer necessary with version 0.6 as it is now working with the astroid 2.0 final release which was created not long ago.