Exceptions.c: use PyCode_NewEmpty() in Py3 instead of PyCode_New() (GH-4479)

With reference to https://github.com/cython/cython/issues/4365#issuecomment-977023011
in Python 3 PyCode_NewEmpty does everything we need to set
exception traceback code objects.

This change is probably only a real improvement on Py3.11 onwards
(where the replacement for PyCode_new is currently pretty slow).
On earlier version it'll probably be fairly similar (maybe one extra allocation for the cline case?)

Cython/Utility/Exceptions.c

@@ -742,31 +742,33 @@ static void __Pyx_AddTraceback(const char *funcname, int c_line,
 static PyCodeObject* __Pyx_CreateCodeObjectForTraceback(
             const char *funcname, int c_line,
             int py_line, const char *filename) {
-    PyCodeObject *py_code = 0;
-    PyObject *py_srcfile = 0;
-    PyObject *py_funcname = 0;
-
+    PyCodeObject *py_code = NULL;
+    PyObject *py_funcname = NULL;
     #if PY_MAJOR_VERSION < 3
+    PyObject *py_srcfile = NULL;
+
     py_srcfile = PyString_FromString(filename);
-    #else
-    py_srcfile = PyUnicode_FromString(filename);
-    #endif
     if (!py_srcfile) goto bad;
+    #endif
+
     if (c_line) {
         #if PY_MAJOR_VERSION < 3
         py_funcname = PyString_FromFormat( "%s (%s:%d)", funcname, $cfilenm_cname, c_line);
+        if (!py_funcname) goto bad;
         #else
         py_funcname = PyUnicode_FromFormat( "%s (%s:%d)", funcname, $cfilenm_cname, c_line);
+        if (!py_funcname) goto bad;
+        funcname = PyUnicode_AsUTF8(py_funcname);
+        if (!funcname) goto bad;
         #endif
     }
     else {
         #if PY_MAJOR_VERSION < 3
         py_funcname = PyString_FromString(funcname);
-        #else
-        py_funcname = PyUnicode_FromString(funcname);
+        if (!py_funcname) goto bad;
         #endif
     }
-    if (!py_funcname) goto bad;
+    #if PY_MAJOR_VERSION < 3
     py_code = __Pyx_PyCode_New(
         0,            /*int argcount,*/
         0,            /*int posonlyargcount,*/
@@ -786,11 +788,16 @@ static PyCodeObject* __Pyx_CreateCodeObjectForTraceback(
         $empty_bytes  /*PyObject *lnotab*/
     );
     Py_DECREF(py_srcfile);
-    Py_DECREF(py_funcname);
+    #else
+    py_code = PyCode_NewEmpty(filename, funcname, py_line);
+    #endif
+    Py_XDECREF(py_funcname);  // XDECREF since it's only set on Py3 if cline
     return py_code;
 bad:
-    Py_XDECREF(py_srcfile);
     Py_XDECREF(py_funcname);
+    #if PY_MAJOR_VERSION < 3
+    Py_XDECREF(py_srcfile);
+    #endif
     return NULL;
 }