README, RPM specfile: stb-tester requires python 2.7

Since 0.12 almost a year ago (specifically, commit 45263fa2) we require
python 2.7 (multiple context expressions in a single `with` statement is
new in 2.7). Since we don't test on 2.6, I don't know how many other
things we do aren't compatible with python 2.6, so at least we should
be honest about it.