Oh cpython/pystate.h introduced this warning, when building Python with clang in release mode:

In file included from Parser/node.c:3:
In file included from ./Include/Python.h:107:
In file included from ./Include/traceback.h:8:
In file included from ./Include/pystate.h:123:
./Include/cpython/pystate.h:217:3: warning: redefinition of typedef 'PyThreadState' is a C11 feature [-Wtypedef-redefinition]
} PyThreadState;
  ^
./Include/pystate.h:27:20: note: previous definition is here
typedef struct _ts PyThreadState;
                   ^