Enable limited C API tests on Python 3.15 on Windows

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Build / build (windows-latest, 3.15t) (pull_request) fails with:

test_pythoncapi_compat_cext.c
"C:\Program Files\Microsoft Visual Studio\18\Enterprise\VC\Tools\MSVC\14.51.36231\bin\HostX86\x64\link.exe" /nologo /INCREMENTAL:NO /LTCG /DLL /MANIFEST:EMBED,ID=2 /MANIFESTUAC:NO /LIBPATH:C:\hostedtoolcache\windows\Python\3.15.0-rc.3\x64-freethreaded\libs /LIBPATH:C:\hostedtoolcache\windows\Python\3.15.0-rc.3\x64-freethreaded /LIBPATH:C:\hostedtoolcache\windows\Python\3.15.0-rc.3\x64-freethreaded\PCbuild\amd64 "/LIBPATH:C:\Program Files\Microsoft Visual Studio\18\Enterprise\VC\Tools\MSVC\14.51.36231\ATLMFC\lib\x64" "/LIBPATH:C:\Program Files\Microsoft Visual Studio\18\Enterprise\VC\Tools\MSVC\14.51.36231\lib\x64" "/LIBPATH:C:\Program Files (x86)\Windows Kits\NETFXSDK\4.8.1\lib\um\x64" "/LIBPATH:C:\Program Files (x86)\Windows Kits\10\lib\10.0.26100.0\ucrt\x64" "/LIBPATH:C:\Program Files (x86)\Windows Kits\10\\lib\10.0.26100.0\\um\x64" /EXPORT:PyInit_test_pythoncapi_compat_cext11 build\temp.win-amd64-cpython-315t\Release\test_pythoncapi_compat_cext.obj /OUT:build\lib.win-amd64-cpython-315t\test_pythoncapi_compat_cext11.cp315t-win_amd64.pyd /IMPLIB:build\temp.win-amd64-cpython-315t\Release\test_pythoncapi_compat_cext11.cp315t-win_amd64.lib
LINK : error LNK2001: unresolved external symbol PyInit_test_pythoncapi_compat_cext11
build\temp.win-amd64-cpython-315t\Release\test_pythoncapi_compat_cext11.cp315t-win_amd64.lib : fatal error LNK1120: 1 unresolved externals