Py3k migration

### Description

- This pull request adds `py3k` support to schematics. As per https://github.com/schematics/schematics/issues/241, this is last but one step. `HISTORY.rst` is updated. 

Thanks @meantheory.

Fixes https://github.com/schematics/schematics/issues/241. Pending final bit is releasing 1.0 to `PyPI`.

@j2labs If anything is broken after merge please blame me :-).