bpo-40941: Fix stackdepth compiler warnings (GH-22377)

Explicitly cast a difference of two pointers to int:
PyFrameObject.f_stackdepth is an int.

Author: vstinner

Python/ceval.c

@@ -1433,9 +1433,9 @@ _PyEval_EvalFrameDefault(PyThreadState *tstate, PyFrameObject *f, int throwflag)
         if (_Py_TracingPossible(ceval2) &&
             tstate->c_tracefunc != NULL && !tstate->tracing) {
             int err;
-            /* see maybe_call_line_trace
+            /* see maybe_call_line_trace()
                for expository comments */
-            f->f_stackdepth = stack_pointer-f->f_valuestack;
+            f->f_stackdepth = (int)(stack_pointer - f->f_valuestack);
 
             err = maybe_call_line_trace(tstate->c_tracefunc,
                                         tstate->c_traceobj,
@@ -2265,7 +2265,7 @@ _PyEval_EvalFrameDefault(PyThreadState *tstate, PyFrameObject *f, int throwflag)
             assert(f->f_lasti >= (int)sizeof(_Py_CODEUNIT));
             f->f_lasti -= sizeof(_Py_CODEUNIT);
             f->f_state = FRAME_SUSPENDED;
-            f->f_stackdepth = stack_pointer-f->f_valuestack;
+            f->f_stackdepth = (int)(stack_pointer - f->f_valuestack);
             goto exiting;
         }
 
@@ -2282,7 +2282,7 @@ _PyEval_EvalFrameDefault(PyThreadState *tstate, PyFrameObject *f, int throwflag)
                 retval = w;
             }
             f->f_state = FRAME_SUSPENDED;
-            f->f_stackdepth = stack_pointer-f->f_valuestack;
+            f->f_stackdepth = (int)(stack_pointer - f->f_valuestack);
             goto exiting;
         }