Minor update to improve quality on development

Since libsass-python 0.19.4, a new minor feature has been added which add the
column position in some error messages. This was breaking tests but without any
impact in Boussole functioning.

For sanity, we added a minor check against libsass-python version in a test to
continue support for libsass-python from 0.18.x to 0.20.x versions. It adds a
new dependency to packaging package only in development requirements.

Finally the Tox configuration has been updated to perform tests against supported
libsass-python versions and flake coverage has been done on tests.