Fix for Python 2.6 syntax

I tried to run the youtrack2youtrack.py tool using python 2.6.6 and received a syntax error.

I discovered this was because the literal set syntax was only back ported in python version 2.7.

I made the syntax fix and thought I'd contribute it.

I'm new to python, GitHub and YouTrack! So don't be afraid to let me know if I've gone wrong somewhere (with code or process).

Cheers.