Ignore inheritance example in class.md

Does not work currently, see PyO3#381

guide/src/class.md

@@ -154,7 +154,7 @@ By default `PyObject` is used as default base class. To override default base cl
 `new` method accepts `PyRawObject` object. `obj` instance must be initialized
 with value of custom class struct. Subclass must call parent's `new` method.
 
-```rust
+```rust,ignore
 # extern crate pyo3;
 # use pyo3::prelude::*;
 # use pyo3::PyRawObject;