[pypy-issue] Issue #2613: Fix the ABI tag (pypy/pypy)

[Antonio Cuni]

New issue 2613: Fix the ABI tag
https://bitbucket.org/pypy/pypy/issues/2613/fix-the-abi-tag

Antonio Cuni:

currently, the pypy ABI tag is fixed to `pypy-41`, as defined in `pypy/module/imp/importing.py`:
```
#DEFAULT_SOABI = 'pypy-%d%d' % PYPY_VERSION[:2]
DEFAULT_SOABI = 'pypy-41'
```

However, this is probably wrong: thanks to the continuous work on cpyext, it is likely that any released pypy is not compatible with the previous ones. I propose to just revert to the old way of making SOABI dependent on the pypy version. Then, when we are stable enough, we can fix it again.

As the comment in `importing.py` suggests, we should also decide what to do with cffi's ABI tag: if we keep it the same as cpyext ABI tag, it means that cffi modules will be recompiled needlessly at each pypy version.
We might want to use a different (more stable) ABI only for cffi stuff, but we need to check whether the existing tools such as `pip` or `setuptools` are able to handle such a thing.