sw/python: fix calls to trtl_close()

Before we were not calling the proper close() function in the destructor
of the TrtlDevice object.

We were also also not checking that the device handle (token) was
defined before using it.

Last but not least, we were not wrapping the device handle as a Python
Ctypes 'c_void_p', which caused Ctypes to convert it to a 32-bit int,
pointing to nowhere in a 64-bit system.