Add missing .. versionadded:: for DataType.is_string (#177)

Signed-off-by: Christoph Berganski <[email protected]>

Author: iksnagreb

src/onnx_ir/_enums.py

@@ -357,7 +357,10 @@ def is_signed(self) -> bool:
         }
 
     def is_string(self) -> bool:
-        """Returns True if the data type is a string type."""
+        """Returns True if the data type is a string type.
+
+        .. versionadded:: 0.1.8
+        """
         return self == DataType.STRING
 
     def __repr__(self) -> str: