bugfix: revert prompt_toolkit to 2.0.6
bugfix: revert prompt_toolkit to 2.0.6

This was introduced by: e9c97072afe9d79276da568ef6791276fddb15d7 (  Use
InputMode.REPLACE_SINGLE (#1208) )

see also: https://github.com/dbcli/pgcli/pull/1197

dbcli/pgcli,dbcli/pgcli