The docs kept suggesting DROP CONSTRAINT constraint_name [ RESTRICT | CASCADE ] but it doesn’t appear to like that — you must pass the constraint_name without the CONSTRAINT keyword. At least on my 8.4.
Don’t do this as the docs might suggest..
ALTER TABLE <table> ALTER COLUMN <column> DROP CONSTRAINT not null;
ALTER TABLE <table> ALTER COLUMN <column> DROP not null; ALTER TABLE my_table ALTER COLUMN my_column DROP not null;