Fix : aggregate method typing to accept list of expressions (#246)

Fixes #209

The type stub for `DuckDBPyRelation.aggregate` incorrectly restricts the
`aggr_expr` parameter to `Expression | str`. However, the DuckDB Python
API
and runtime behavior also support passing a list of `Expression` objects
for
multiple aggregations.

The fix extends the type annotation to include `list[Expression]`,
aligning
the stub definition with the actual supported API behavior.
``` 
   def aggregate(
        self, aggr_expr: Expression | str | list[Expression], group_expr: Expression | str = ""
    ) -> DuckDBPyRelation: ...
```

Author: evertlammerts

_duckdb-stubs/__init__.pyi

@@ -461,7 +461,9 @@ class DuckDBPyRelation:
     def __getattr__(self, name: str) -> DuckDBPyRelation: ...
     def __getitem__(self, name: str) -> DuckDBPyRelation: ...
     def __len__(self) -> int: ...
-    def aggregate(self, aggr_expr: Expression | str, group_expr: Expression | str = "") -> DuckDBPyRelation: ...
+    def aggregate(
+        self, aggr_expr: Expression | str | list[Expression], group_expr: Expression | str = ""
+    ) -> DuckDBPyRelation: ...
     def any_value(
         self, column: str, groups: str = "", window_spec: str = "", projected_columns: str = ""
     ) -> DuckDBPyRelation: ...