PEP 739 / `build-details.json`: `c_api.headers` does not include the `pythonX.Y` directory

[mgorny]

Bug report

Bug description:

Sorry I didn't notice this while fixing #134455 but c_api.headers key contains the path to top-level system include directory, i.e.:

{
  "c_api": {
    "headers": "/usr/local/include",
    "pkgconfig_path": "/usr/local/lib/pkgconfig"
  }
}

while according to the examples in PEP 739, it should contain the path to the pythonX.Y directory:

Examples

/usr/include/python3.14
include/python3.14
etc.

CPython versions tested on:

3.14, CPython main branch

Operating systems tested on:

Linux

Linked PRs

• gh-134632: Fix build-details.json to use INCLUDEPY path #134633