override get_select_precolumns() in StrSQLCompiler

override get_select_precolumns() in StrSQLCompiler

Fixed issue where :meth:`_sql.Select.get_final_froms` would emit a
deprecation warning when the statement made use of the PostgreSQL-specific
expression argument to :meth:`_sql.Select.distinct`; the same spurious
warning would be emitted when stringifying such a statement without
explicitly using a PostgreSQL dialect.  The fix ensures that this 1.4-era
warning is suppressed under both 2.0 and 2.1.

Note that under SQLAlchemy 2.1, passing an expression to
:meth:`_sql.Select.distinct` is deprecated overall, and is replaced by a
new PostgreSQL-specific construct (see :ticket:`12342`).

Fixes: #13396
Change-Id: I587e24aa7016c56b1d5bfe1048c4b6eb809dfb30