PostgreSQL dialect fails to parse `IN` with union

[adamchainz]

Minimal example:

SELECT 1 FROM t WHERE t.id IN ((SELECT id FROM t) UNION (SELECT id FROM t))

Currently, this fails with:

ParserError("Expected: ), found: UNION at Line: 1, Column: 51")

I checked this works on PostgreSQL shell:

postgres=# create table t(id int);
CREATE TABLE
postgres=# SELECT 1 FROM t WHERE t.id IN ((SELECT id FROM t) UNION (SELECT id FROM t));
 ?column?
----------
(0 rows)

And it fails to parse on SQLite:

sqlite> create table t(id int);
sqlite> SELECT 1 FROM t WHERE t.id IN ((SELECT id FROM t) UNION (SELECT id FROM t));
Parse error: near "UNION": syntax error
  SELECT 1 FROM t WHERE t.id IN ((SELECT id FROM t) UNION (SELECT id FROM t));
                                      error here ---^