CPython: merge expressions of interpreters

Each time a new major/minor version of CPython was released, a new
expression would be written, typically copied from the previous release.
Often fixes are only made in the current/latest release. By merging the
expressions it's more likely that modifications end up in all versions,
as is likely intended.

This commit introduces one expression for Python 3, and another for 2.7.
These two may also be merged, but it will result in a lot of extra
conditionals making the expression harder to follow.

A common passthru is introduced for CPython and PyPy.

python 2.7: use common passthru