Regression in f387d024fc.
Just like `OrderByList` the `ExpressionList` expression used to wrap
`Window.partition_by` must implement `get_group_by_cols` to ensure the
necessary grouping when mixing window expressions with aggregate
annotations is performed against the partition members and not the
partition expression itself.
This is necessary because while `partition_by` is implemented as
a source expression of `Window` it's actually a fragment of the WINDOW
expression at the SQL level and thus it should result in a group by its
members and not the sum of them.
Thanks ElRoberto538 for the report.
e76cc93b01