However, depriving a user from the freedom to do as he pleases (in this case, dictating what environment to use) is a disastrous mistake. That's a valid criticism, but maybe python could still warn about the mismatch, even if it would not reject the program by default.
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. I agree, but there are also environments that I can determine not being able to run my code, and then I could warn on those. As to CPython, etc., maybe those can match against a "normalized" version number that they claim to be compatible with.
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. I guess you are right about that. And for the really important cases, I suppose performing a check in the makefile, or, if everything else fails, directly in the code (yuck) should suffice.