Add PEP 573 to the stable ABI

[encukou]

BPO	42171
Nosy	@ncoghlan, @scoder, @vstinner, @encukou, @corona10, @miss-islington, @shihai1991
PRs	bpo-42171: Add PEP573-related items to the limited API #23009

^{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 2020-11-10.15:16:08.897>
created_at = <Date 2020-10-27.14:50:49.749>
labels = ['expert-C-API', '3.10']
title = 'Add PEP 573 to the stable ABI'
updated_at = <Date 2020-11-10.15:16:08.896>
user = 'https://github.com/encukou'

bugs.python.org fields:

activity = <Date 2020-11-10.15:16:08.896>
actor = 'petr.viktorin'
assignee = 'none'
closed = True
closed_date = <Date 2020-11-10.15:16:08.897>
closer = 'petr.viktorin'
components = ['C API']
creation = <Date 2020-10-27.14:50:49.749>
creator = 'petr.viktorin'
dependencies = []
files = []
hgrepos = []
issue_num = 42171
keywords = ['patch']
message_count = 5.0
messages = ['379762', '379765', '379766', '380664', '380675']
nosy_count = 7.0
nosy_names = ['ncoghlan', 'scoder', 'vstinner', 'petr.viktorin', 'corona10', 'miss-islington', 'shihai1991']
pr_nums = ['23009']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue42171'
versions = ['Python 3.10']

[encukou]

The following PEP-573 were not added to the Windows list of stable ABI symbols (PC/python3dll.c):

• PyModule_AddType
• PyType_FromModuleAndSpec
• PyType_GetModule
• PyType_GetModuleState

I'd like to add them.

Also, the PEP introduces the (METH_FASTCALL | METH_KEYWORDS | METH_METHOD) calling convention, but METH_FASTCALL is not part of the stable ABI (and it other uses are not suitable for stable ABI).
I propose adding a new value, METH_METHOD_ARGS_KWD (name up for bikeshedding), which would be equal to (METH_FASTCALL | METH_KEYWORDS | METH_METHOD) but part of the limited API.

Does that sound like a good idea?

On the sprint, Steve mentioned that it would be possible to backport this to Python 3.9.1, even though it would mean a weird minor-version requirement.
I no longer think that's a good idea: fixing the stable ABI will be a longer-term project, and I don't want to fight issues in a 3.9 backport.

FWIW, for the longer-term project, I started a brainstorming repo at https://github.com/encukou/abi3 (Yes, it has some overlap with Victor's more general notes at https://pythoncapi.readthedocs.io/ )
But I'd like to get the additions above in to unblock bpo-42111 before I start on improving the stable ABI in general.

[encukou]

Correction: PyModule_AddType is not from PEP-573. I'd like to add it anyway.

[corona10]

I'd like to add it anyway.
Nice ;)

[miss-islington]

New changeset 0b9c4c6 by Petr Viktorin in branch 'master':
bpo-42171: Add PEP-573-related items to the limited API (GH-23009)
0b9c4c6

[encukou]

Added.