[repo] don't set cnxset when getting the session to close it and avoid warning about that. session._cnx.rollback won't do anything if it's not set and a newer connection is now created to run the session_close hook, so there is not need to set cnxset. Notice we have to take care about not trying to rollback the underlying cnx when the session is not 'session_handled' (i.e. bw compat mode). This case only occurs due to hackish things to keep bw compat in testlib.

usr/lib/PY_VERSION/*-packages/cubicweb/server/
usr/lib/PY_VERSION/*-packages/cubicweb/hooks/
usr/lib/PY_VERSION/*-packages/cubicweb/sobjects/
usr/lib/PY_VERSION/*-packages/cubicweb/schemas/
usr/share/cubicweb/migration/