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.

Require cubicweb 3.20 for cubicweb.utils.admincnx.
This means mboximport no longer works with a dbapi connection, so the
ccplugin is broken.

Operations get a cnx instead of a session.

Use int instead of typed_eid.

cubicweb.web.uicfg is now cubicweb.web.views.uicfg, and cw_set replaces
set_relations.