- The :attr:`.Column.key` attribute is now used as the source of
anonymous bound parameter names within expressions, to match the
existing use of this value as the key when rendered in an INSERT
or UPDATE statement.   This allows :attr:`.Column.key` to be used
as a "substitute" string to work around a difficult column name
that doesn't translate well into a bound parameter name.   Note that
the paramstyle is configurable on :func:`.create_engine` in any case,
and most DBAPIs today support a named and positional style.
fixes #3245
- in lieu of adding a new system of translating bound parameter names
for psycopg2 and others, encourage users to take advantage of positional
styles by documenting "paramstyle".   A section is added to psycopg2
specifically as this is a pretty common spot for named parameters
that may be unusually named.  fixes #3246.