gh-107211: No longer export internal variables (#107218)

No longer export these 5 internal C API variables:

* _PyBufferWrapper_Type
* _PyImport_FrozenBootstrap
* _PyImport_FrozenStdlib
* _PyImport_FrozenTest
* _Py_SwappedOp

Fix the definition of these internal functions, replace PyAPI_DATA()
with PyAPI_FUNC():

* _PyImport_ClearExtension()
* _PyObject_IsFreed()
* _PyThreadState_GetCurrent()