Use latin encoding to store filestrings

I think this makes get-pip.py a little smaller as well, because instead of 

txt->b64->bz2->b64 it's just txt->bz2->b64

latin1 essentially allows py3 to decode whole bytes instead of just 127 values, lol.


works for me on py33 and py27, toothrot said it worked for him on py33 windows venv, and user reports it solves py32 for #1503

I thought `temp_dir` worked better inside the try instead of in `unpack()`, makes `unpack()` a little more invariant.