update type hints for Python 3.9 compatibility

Author: msamsami

crudadmin/core/schemas/timestamp.py

@@ -11,15 +11,15 @@ class TimestampSchema(BaseModel):
     updated_at: Optional[datetime] = Field(default=None)
 
     @field_serializer("created_at")
-    def serialize_dt(self, created_at: datetime | None, _info: Any) -> str | None:
+    def serialize_dt(self, created_at: Optional[datetime], _info: Any) -> Optional[str]:
         if created_at is not None:
             return created_at.isoformat()
         return None
 
     @field_serializer("updated_at")
     def serialize_updated_at(
-        self, updated_at: datetime | None, _info: Any
-    ) -> str | None:
+        self, updated_at: Optional[datetime], _info: Any
+    ) -> Optional[str]:
         if updated_at is not None:
             return updated_at.isoformat()
         return None