also ensure the requirements-test.txt is included in the sdist

so it can be used in other cubes.

we add Twisted since we use CubicWebServerTC. Ideally we would like to have
this later use wsgi instead of twisted, but hey.

and make it work with cw 3.22

been lost in cw->cwclientlib translation

The mailbox module wants to seek into the file to build its toc, so give
it a BytesIO if our input stream isn't seekable.

also upgrade to dh_python2 and other several packaging updates/fixes.

also update the setup.py file and make bin/cw-mboximport managed by setup.py.
We still provide this as a biin/ script instead of an entry_point since we
still build our deb packages without setuptools...

Closes #4616894

So far this was only enforced by the MBOXImporter.

Taking information from headers and turning it into our schema's
attributes/relations shouldn't depend on the import script.

To make parsing easier on the hooks side, the 'headers' attribute is
now expected to be an undecoded ascii-only string.