pycore_pystate.h no longer redefines PyThreadState_GET() (GH-28921)

Redefining the PyThreadState_GET() macro in pycore_pystate.h is
useless since it doesn't affect files not including it. Either use
_PyThreadState_GET() directly, or don't use pycore_pystate.h internal
C API. For example, the _testcapi extension don't use the internal C
API, but use the public PyThreadState_Get() function instead.

Replace PyThreadState_Get() with _PyThreadState_GET(). The
_PyThreadState_GET() macro is more efficient than PyThreadState_Get()
and PyThreadState_GET() function calls which call fail with a fatal
Python error.

posixmodule.c and _ctypes extension now include <windows.h> before
pycore header files (like pycore_call.h).

_PyTraceback_Add() now uses _PyErr_Fetch()/_PyErr_Restore() instead
of PyErr_Fetch()/PyErr_Restore().

The _decimal and _xxsubinterpreters extensions are now built with the
Py_BUILD_CORE_MODULE macro defined to get access to the internal C
API.

Python/context.c

@@ -728,7 +728,7 @@ static int
 contextvar_set(PyContextVar *var, PyObject *val)
 {
     var->var_cached = NULL;
-    PyThreadState *ts = PyThreadState_Get();
+    PyThreadState *ts = _PyThreadState_GET();
 
     PyContext *ctx = context_get();
     if (ctx == NULL) {