Do not work in python3

The package does not seem to work in Python3, but it does in python2.
The package does not have a version either, which is usefull when looking into changes.
```
    import cosmolopy.perturbation as pert
  File "<me>/.local/lib/python3.6/site-packages/cosmolopy/__init__.py", line 88, in <module>
    import constants as cc
ModuleNotFoundError: No module named 'constants'
```