Fixed Python 3.1 and 3.2 support by removing unicode literals

This fix comes at a price - it introduces tiny but unavoidable compatibility module with one function replacing unicode literal which doesn't work in Python 3.1-3.2 (brought back in Python 3.3 by PEP 414). I believe though, that support for two modern Python versions, packaged in many of current stable Linux distributions, is worth it.

The compatibility code has been shamelessly copied from `zope.schema`.