GCC style fallthrough attribute is used with clang versions that don't support it

[jmroot]

Bug report

Bug description:

Include/pyport.h currently only guards use of __attribute__((fallthrough)) with _Py__has_attribute(fallthrough). Clang versions prior to 10 do not support this syntax, but do claim to support the fallthrough attribute (because they support the C++11 style [[fallthrough]]).

CPython versions tested on:

3.14

Operating systems tested on:

macOS

Linked PRs

• gh-128902: Fix check for fallthrough attribute support #128903

[ZeroIntensity]

Looks like this was done in #121044

cc @sobolevn @vstinner

[ned-deily]

Fixed in #128903 edf8033 for 3.14.0a5