gh-99202: Fix extension type from documentation for compiling in C++20 mode

[jpn--]

As noted in #99202, C++20 added support for designated initializers and fails to compile if you mix named and unnamed initializers.

The documentation currently includes a section "Defining Extension Types: Tutorial" that mixed named and unnamed initializers, so it will not compile as shown.

This PR adds names on the header macros, so they do now compile in C++20 mode. The change is only made in the demo documentation, not any "real" code.

• Issue: Extension type from documentation doesn't compile in C++20 mode #99202

[bedevere-bot]

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[ghost]

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[arhadthedev]

@vstinner @encukou @erlend-aasland @bruno-at-bareos as participants of the parent issue.

[vstinner]

LGTM. IMO it's fine to use this syntax in the documentation for new extensions.

@encukou: What do you think?

Python 3.11 itself now targets C11: https://peps.python.org/pep-0007/#c-dialect Python still supports building extensions with older C standards and various C++ standards.

[encukou]

This actually looks great! I'm surprised that there aren't any obvious downsides!

IMO it would be nice to introduce PyVarObject_HEAD_BASE macros, named to match the field name, which wouldn't include the comma. But that's a separate issue: it would still make sense to use that with .ob_base =

Victor, do you want to merge? (I can't commit to watching/fixing buildbots this week.)

[vstinner]

Merged, thanks!