Fix : aggregate method typing to accept list of expressions

[Narwhal-fish]

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: ...

[evertlammerts]

Thanks, LGTM! Can you rebase on v1.4-andium?

[Narwhal-fish]

Thanks, LGTM! Can you rebase on v1.4-andium?

Sure!