Add Select.get_final_froms() (#217)

https://docs.sqlalchemy.org/en/14/core/selectable.html#sqlalchemy.sql.expression.Select.get_final_froms

Author: rouge8

sqlalchemy-stubs/sql/selectable.pyi

@@ -581,6 +581,7 @@ class Select(
     def inner_columns(self) -> Any: ...
     def is_derived_from(self, fromclause: Any) -> bool: ...
     def get_children(self, **kwargs: Any) -> Iterable[Any]: ...  # type: ignore[override]
+    def get_final_froms(self) -> Any: ...
     def add_columns(self: _SEL, *columns: Any) -> _SEL: ...
     def column(self: _SEL, column: Any) -> _SEL: ...
     def reduce_columns(self: _SEL, only_synonyms: bool = ...) -> _SEL: ...