Use new C-API function to get the current frame on Python 3.11 alpha (GH-4427)

Use the accessor function instead of the direct struct member (which is now the wrong type).

Closes https://github.com/cython/cython/issues/4416

Cython/Utility/Coroutine.c

@@ -719,8 +719,7 @@ PyObject *__Pyx_Coroutine_SendEx(__pyx_CoroutineObject *self, PyObject *value, i
 
     exc_state = &self->gi_exc_state;
     if (exc_state->exc_type) {
-        #if CYTHON_COMPILING_IN_PYPY || PY_VERSION_HEX >= 0x030B00A1
-        // FIXME: https://bugs.python.org/issue44590 - Python 3.11 changed the type of frame
+        #if CYTHON_COMPILING_IN_PYPY
         // FIXME: what to do in PyPy?
         #else
         // Generators always return to their most recent caller, not
@@ -729,9 +728,14 @@ PyObject *__Pyx_Coroutine_SendEx(__pyx_CoroutineObject *self, PyObject *value, i
             PyTracebackObject *tb = (PyTracebackObject *) exc_state->exc_traceback;
             PyFrameObject *f = tb->tb_frame;
 
-            Py_XINCREF(tstate->frame);
             assert(f->f_back == NULL);
+            #if PY_VERSION_HEX >= 0x030B00A1
+            // FIXME: unclear what to do if PyThreadState_GetFrame fails
+            f->f_back = PyThreadState_GetFrame(tstate);
+            #else
+            Py_XINCREF(tstate->frame);
             f->f_back = tstate->frame;
+            #endif
         }
         #endif
     }