python file versioning

[kartik]

2 enable non-backward compatible changes to be made to the language or
the standard library (or other libraries, for that matter), i feel
there should be a way to version python files. the current __future__
system suffers from changing behaviour in the absence of the imports,
from version to version of the interpreter. this means that i can't
simply run a python file without knowing which version of the
interpreter it was developed against.

with such a versioning system, non-backward compatible changes can be
made to libraries without breaking code. (the __future__ mechanism
seems to cover only the language)

at some time in the future (not __future__!), we could allow
side-by-side execution of different versions of libraries on the same
interpreter.

-kartik