[3.15] gh-140550: Fix PyABIInfo_VAR macro: avoid ";"

[vstinner]

Remove ";" from PyABIInfo_VAR macro definition, since "PyABIInfo_VAR(abi_info);" added two ";;" which is illegal in C++03.

• Issue: Implement PEP 793 – PyModExport #140550

[vstinner]

@encukou: I found this bug while enabling limited C API tests on Free Threading in test_cppext for PR gh-157493. IMO we should fix this issue in Python 3.15.0. Otherwise, developers may start writing extensions as PyABIInfo_VAR(abi_info) (without ;) which is currently accepted.

I checked the documentation: PyABIInfo_VAR is always used as PyABIInfo_VAR(abi_info); (with ;).

[encukou]

I agree. Thanks for the catch!

It's @hugovk's call though. For context:
With the fix, existing extension modules will work, but some existing ones might fail to compile until they get a small source fix.
All docs show the “correct” way to use PyABIInfo_VAR, but, projects that compile with strict C++03 compliance settings might have adjusted and will now need adjusting back.

IMO, for C-wielding early adopters, a NEWS entry is enough.

[hugovk]

Yes, let's have this fixed for 3.15.0, please add the NEWS entry.

Should this go into main first?

[vstinner]

Yes, let's have this fixed for 3.15.0, please add the NEWS entry.

I added a NEWS entry.

Should this go into main first?

I extracted the PyABIInfo_VAR macro fix from my larger PR gh-157493. The main change is under review. (I just fixed test_cppext when using clang.)

[encukou]

Should this go into main first?

IMO it should, to keep the change & NEWS together, and make it easier to revert if there's some unforeseen weirdness. I sent #157539.

(If you think keeping this in the bigger change is better I won't object, but then add the NEWS entry there.)

[hugovk]

Thanks both, both merged!