[5.0.x] Fixed #34987 -- Fixed queryset crash when mixing aggregate and window annotations.

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.
Backport of e76cc93b01 from main

docs/releases/4.2.8.txt

@@ -16,3 +16,7 @@ Bugfixes
 * Fixed a regression in Django 4.2 that caused a crash of
   ``QuerySet.aggregate()`` with aggregates referencing other aggregates or
   window functions through conditional expressions (:ticket:`34975`).
+
+* Fixed a regression in Django 4.2 that caused a crash when annotating a
+  ``QuerySet`` with a ``Window`` expressions composed of a ``partition_by``
+  clause mixing field types and aggregation expressions (:ticket:`34987`).