Fix wrong type annotation (#1235)

Co-authored-by: Kazuhiro Sera <[email protected]>

Author: ehdgua01

slack_sdk/models/blocks/block_elements.py

@@ -117,7 +117,9 @@ def parse(cls, block_element: Union[dict, "BlockElement"]) -> Optional[Union["Bl
             return None
 
     @classmethod
-    def parse_all(cls, block_elements: Sequence[Union[dict, "BlockElement"]]) -> List["BlockElement"]:
+    def parse_all(
+        cls, block_elements: Sequence[Union[dict, "BlockElement", TextObject]]
+    ) -> List[Union["BlockElement", TextObject]]:
         return [cls.parse(e) for e in block_elements or []]