scripts/openbabel-python.i: Revert shared_ptr changes

[Reinis]

This reverts part of ce178cb.

[Reinis]

Fixes regression:

~$ python
Python 3.3.2 (default, Nov 21 2013, 05:56:52) 
[GCC 4.8.2] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import openbabel as ob
>>> m = ob.OBMol()
>>> m += ob.OBMol()
>>> m += ob.OBMol()
Segmentation fault

[ghutchis]

I'd like a more permanent solution, but I agree with this. (The Python bindings would not compile on Mac before this change.)