Fix a compiler warning.

Add a missing cast to fix the following build-time compiler warning:

```
/usr/bin/clang -fno-common -dynamic -DNDEBUG -g -fwrapv -O3 -Wall -Wstrict-prototypes -pipe -Os -I/opt/local/Library/Frameworks/Python.framework/Versions/3.2/include/python3.2m -c traits/ctraits.c -o build/temp.macosx-10.9-x86_64-3.2/traits/ctraits.o -DNDEBUG=1 -O3
In file included from traits/ctraits.c:27:
traits/py2to3.h:211:27: warning: incompatible pointer types passing 'PyDictObject *' (aka 'struct _dictobject *') to parameter of type 'PyObject *' (aka 'struct _object *') [-Wincompatible-pointer-types]
    return PyDict_GetItem(dict, name);
                          ^~~~
/opt/local/Library/Frameworks/Python.framework/Versions/3.2/include/python3.2m/dictobject.h:110:49: note: passing argument to parameter 'mp' here
PyAPI_FUNC(PyObject *) PyDict_GetItem(PyObject *mp, PyObject *key);
                                                ^
1 warning generated.
```