Switch to using the stable ABI

[ncoghlan]

It should be possible to implement asciicompat using the stable ABI, which will make it easier to maintain (a single binary will work on all Python version 3.2+).

The main changes needed will to define PY_LIMITED_API before importing python.h and to switch to use PyType_FromSpec rather than the legacy type declaration model.

The docs about this are rather suboptimal (see http://bugs.python.org/issue20224).

[jeamland]

How does one implement the buffer API with that approach?

[jeamland]

Also how do I declare a subtype of unicode without PyType_FromSpecWithBases which appears to be limited to 3.3 and up?

[ncoghlan]

You can't support 3.2 with it, but I suspect we'll end up needing 3.3+ for
interop bug fixes anyway.

However, it's probably worth waiting and seeing how far we can get while
supporting 3.2.

[ncoghlan]

Implementing the buffer API should be like implementing any other slot. The
best docs for that are currently in PEP 384.

[ncoghlan]

Ah, now I remember why 3.3 is already the minimum version - we need the
compact internal layout from PEP 393 in order to easily expose the array of
bytes.

[jeamland]

From PEP 384:

The buffer interface (type Py_buffer, type slots bf_getbuffer and bf_releasebuffer, etc) has been omitted from the
ABI, since the stability of the Py_buffer structure is not clear at this time. Inclusion in the ABI can be considered in
future releases.

And from CPython 3.4.0b2, Include/typeslots.h:

/* Disabled, see #10181 */
#undef Py_bf_getbuffer
#undef Py_bf_releasebuffer

[ncoghlan]

Oh, damn - we forgot to change that after Stefan fixed the back end in
Python 3.3. OK, static type it is, then. Could you do me a favour and file
an issue about that for Python 3.5?