Error while using Sharedbuffers on pure python mode

If you don't cythonize sharedbuffers before running the code the following error take place:

```
/home/tzulberti/workspace/sharedbuffers/sharedbuffers/mapped_struct.py in <module>()
     40     from types import BufferType as buffer
     41 
---> 42 assert Py_LT == 0
     43 assert Py_LE == 1
     44 assert Py_EQ == 2

NameError: name 'Py_LT' is not defined
```