Update StringIO/BytesIO imports in run_tests.py

This is enough to run this file unmodified on both Python 2.6+
and Python 3.3
Implement StringIO import via Bio._py3k module

The goal here is to facilitate writing code which can be
run on both Python 2 and Python 3 without alteration via
2to3 (and fixing these imports currently needs 2to3).
Import StringIO via _py3k module in tests