[WIP] Add a new experimental _Py_CAPI2 API

[vstinner]

BPO	35206
Nosy	@vstinner
PRs	[WIP] bpo-35206: Add new experimental _Py_CAPI2 API #10443

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2018-11-22.16:27:53.358>
created_at = <Date 2018-11-10.01:37:33.708>
labels = ['interpreter-core', '3.8']
title = '[WIP] Add a new experimental _Py_CAPI2 API'
updated_at = <Date 2018-11-22.16:27:53.356>
user = 'https://github.com/vstinner'

bugs.python.org fields:

activity = <Date 2018-11-22.16:27:53.356>
actor = 'vstinner'
assignee = 'none'
closed = True
closed_date = <Date 2018-11-22.16:27:53.358>
closer = 'vstinner'
components = ['Interpreter Core']
creation = <Date 2018-11-10.01:37:33.708>
creator = 'vstinner'
dependencies = []
files = []
hgrepos = []
issue_num = 35206
keywords = ['patch']
message_count = 3.0
messages = ['329583', '329584', '330268']
nosy_count = 1.0
nosy_names = ['vstinner']
pr_nums = ['10443']
priority = 'normal'
resolution = 'rejected'
stage = 'resolved'
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue35206'
versions = ['Python 3.8']

[vstinner]

Add a new experimental "_Py_CAPI2" API: opt-in which doesn't leak implementation details and replace macros with function calls.

Attached PR modified PyTuple_GET_ITEM() if _Py_CAPI2 is defined.

With this API, PyTuple_GET_ITEM() macro becomes a function call and
the implementation uses assertions to check if the first argument is
a tuple and that the index is valid. It should help to investigate
bugs when Python is compiled in debug mode.

This issue implements the idea that I proposed on python-dev:
https://mail.python.org/pipermail/python-dev/2018-November/155702.html

[vstinner]

I previously tried a very similar patch but reusing Py_DEBUG define, rather than adding a new _Py_CAPI2 define: bpo-35199. I abandonned it because it was backward incompatible. This change is backward compatible: the new API is not compatible with the current API, but the change doesn't impact the default API which remains unchanged.

[vstinner]

According to discussions on python-dev, a PEP will be needed first. Moreover, I prefer to make some progress on https://bugs.python.org/issue35134 before. I close the issue.