It would be very nice if the Python version was something specified in the project manifest. Don't know what is the popular build tool in python, but something like specifying the node.js version in the package.json file would be great.
I think the reason it never works like that is that the language is written before the package manager and VM/compiler.
In Clojure it's the other way around - the JVM and the package managers were there before the language. The Clojure compiler is a library that you include in your project.clj file like any other lib. Hence you can specify language version per project. That's a feature I would like to have in other programming environments.
Are you suggesting that package versions should be explicitly provided as part of import statements? Or that import statements should be mangled depending on the versions specified in setup.py?