bpo-45482: Rename namespaceobject.h to pycore_namespace.h (GH-28975)

Rename Include/namespaceobject.h to
Include/internal/pycore_namespace.h.

The _testmultiphase extension is now built with the
Py_BUILD_CORE_MODULE macro defined to access _PyNamespace_Type.

object.c: remove unused "pycore_context.h" include.