g.next should be next(g) for python 3.x

[pschmidtke]

Hey, just wanted to point out that the documentation for pybel isn't up to date for newer python versions (tried with python 3.8.

https://github.com/openbabel/documentation/blob/master/UseTheLibrary/Python_Pybel.rst for instance, states to use read_file().next() to access the molecule from the generator object. This should be:
next(read_file())

[cclauss]

Also see #20 for other Python 3 issues.