__len__binding don't work in 0.17.x versions

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Binding of the __len__ function don't work anymore in pybindgen version >= 0.17.x when used with python2.7.

If I want to bind the __len__ method with a line looking like this:

mb.add_method('size', retval('int'), [], is_const=True, custom_name='__len__')
(full code available there: https://github.com/jorisv/Eigen3ToPython/blob/master/src/generate.py#L61)

The following code will fail:

```
import eigen3
a = eigen3.Vector3d()
len(a)
---------------------------------------------------------------------------
SystemError Traceback (most recent call last)
<ipython-input-3-af8c77e09569> in <module>()
----> 1 len(a)

SystemError: ../Objects/longobject.c:371: bad argument to internal function
```

Applying the joint patch fix the issue by forcing the use of PyLong_asSsize_t in python3 and PyInt_asSsize_t in python2.