Basic support for Python 3.3

The following changes are required to be able to run `peep install -r requirements.txt` under Python 3.3, without encountering a syntax issue, or type issue, or function/method rename.

Steps which were run by way of testing:
* create `requirements.txt`
* add a pinned package to it (`Django==1.6.2`)
* Run `peep install -r requirements.txt` until enough errors had been fixed to get to the sha256 printout.
* Added the sha256 comment into the `requirements.txt` file
* Re-run to check it installed correctly having matched the sha256.