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

* gh-134632: Fix `build-details.json` to use `INCLUDEPY` path

Fix ``build-details.json`` generation to use ``INCLUDEPY``, in order to
reference the ``pythonX.Y`` subdirectory of the include directory, as
required in :pep:`739`, instead of the top-level include directory.

* test_build_details: Add tests for the c_api section

* test_build_details: Expect pkgconfig for CPython unconditionally

Tools/build/generate-build-details.py

@@ -75,7 +75,7 @@ def generate_data(schema_version: str) -> collections.defaultdict[str, Any]:
     PY3LIBRARY = sysconfig.get_config_var('PY3LIBRARY')
     LIBPYTHON = sysconfig.get_config_var('LIBPYTHON')
     LIBPC = sysconfig.get_config_var('LIBPC')
-    INCLUDEDIR = sysconfig.get_config_var('INCLUDEDIR')
+    INCLUDEPY = sysconfig.get_config_var('INCLUDEPY')
 
     if os.name == 'posix':
         # On POSIX, LIBRARY is always the static library, while LDLIBRARY is the
@@ -123,7 +123,7 @@ def generate_data(schema_version: str) -> collections.defaultdict[str, Any]:
     if has_static_library:
         data['libpython']['static'] = os.path.join(LIBDIR, LIBRARY)
 
-    data['c_api']['headers'] = INCLUDEDIR
+    data['c_api']['headers'] = INCLUDEPY
     if LIBPC:
         data['c_api']['pkgconfig_path'] = LIBPC