Simplified SQLCompiler.get_group_by() a bit.

django/db/models/sql/compiler.py

@@ -148,9 +148,7 @@ class SQLCompiler:
                 continue
             if alias:
                 aliased_exprs[expr] = alias
-            cols = expr.get_group_by_cols()
-            for col in cols:
-                expressions.append(col)
+            expressions.extend(expr.get_group_by_cols())
         if not self._meta_ordering:
             for expr, (sql, params, is_ref) in order_by:
                 # Skip references to the SELECT clause, as all expressions in