the following 2 statements in the Spanner DDL window do the trick :
ALTER TABLE test ADD COLUMN test STRING(MAX);
ALTER TABLE test ALTER COLUMN test STRING(MAX) NOT NULL;

But there is no native way of doing it in SQLAlchemy
The workaround we currently use is to manually edit the migration file to add the two above statements using op.execute(). Tt would be nice if it worked out-of-the-box