trytond depends on psycopg 2.7.0
Since #10245 (closed), we use psycopg2.sql module which was introduced only in version 2.7.0 of psycopg2. The problem is that there is no version psycopg2cffi released which contains this module: https://github.com/chtd/psycopg2cffi/commit/bca6b9c4a21b219845e150eab6cc77ce18d17f5c. So I propose to set the dependency for the future release which will make the installation failing but currently it is already failing. Users who really need it may copy the sql.py file to the psycopg2cffi installation.