1) It's not version number that matters, it's functionality. There are environments you can't even imagine where your code can be run. There are Python builds and flavours outside the CPython's version sequence.
Very true, but other Python flavors commonly specify what features they version of CPython's features they support. eg "Jython 2.5 implements the same language as CPython 2.5", "IronPython 2.7 Alpha 1 was released, supporting Python 2.7 features"
2) ... The right place to tell a user the environment your code was written for is documentation. A user just needs to know where to rely on you and where to take responsibility his/herself.
Well if a project you want to use doesn't specify what version they support pretty well, taking responsibility yourself is potentially time consuming... I for one like the idea of having some automated systems take care of that, to lower the cost for projects to support a wider range of python versions. Establishing an idiom like was done for __future__ is only the start of that, and may not even be strictly necessary but it would be a good start. -Tony