Use new-style Python classes

[orbeckst]

Use new style-classes in

• package/MDAnalysis/core/Selection.py
• package/MDAnalysis/lib/NeighborSearch.py

Notes on How to fix.

See the issue Use new-style Python classes at https://www.quantifiedcode.com/app/project/b993444b512c4f9ab23f9f20e6af7479?branch=origin%2Fdevelop but note that one should not change anything in the file libxdrfile2.py because it is auto-generated by SWIG.