Fix RichTextElementParts.Date.timestamp to be an int (#1560)

Author: PaarthShah

slack_sdk/models/blocks/block_elements.py

@@ -2104,7 +2104,7 @@ def attributes(self) -> Set[str]:
         def __init__(
             self,
             *,
-            timestamp: str,
+            timestamp: int,
             format: str,
             url: Optional[str] = None,
             fallback: Optional[str] = None,