CLN: Refactored so that there is no longer a need for 2to3

The codebase has been refactored so that it can exist in a single, unified base.
The strategy closely followed six and pandas.compat, but does not involve any further
dependencies.

The compatability location has been renamed form compatnp, which housed compatnp.py3k, to
just compat, and the main py3k compatability files are in __init__.py so that they can
be directly accessed