Porting Py3.8/Py3.9:

Since Py3.3: 'Py_ssize_t PyUnicode_GetSize(PyObject*)' is deprecated [-Wdeprecated-declarations]
Since Py3.9: 'PyObject* PyEval_CallObjectWithKeywords(PyObject*, PyObject*, PyObject*)' is deprecated [-Wdeprecated-declarations]
Since Py3.9: 'void PyEval_InitThreads()' is deprecated [-Wdeprecated-declarations]

src/Gui/WidgetFactory.cpp

@@ -1677,7 +1677,11 @@ void SignalConnect::onExecute()
 
     /* Time to call the callback */
     arglist = Py_BuildValue("(O)", myResource);
+#if PY_VERSION_HEX < 0x03090000
     result = PyEval_CallObject(myCallback, arglist);
+#else
+    result = PyObject_CallObject(myCallback, arglist);
+#endif
     Py_XDECREF(result);
     Py_DECREF(arglist);
 }