Update swigpyrun.in for Python 3.8

In Python 3.8 some functions related to thread state were intentionally made private.

This fix (specific to FreeCAD) was suggested here:

https://bugs.python.org/issue35886

src/Base/swigpyrun.inl

@@ -72,7 +72,7 @@ void cleanupSWIG_T(const char* TypeName)
 
     PyObject *module, *dict;
     PyInterpreterState *interp = PyThreadState_GET()->interp;
-    PyObject *modules = interp->modules;
+    PyObject *modules = PyImport_GetModuleDict();
     module = PyDict_GetItemString(modules, "__builtin__");
     if (module != NULL && PyModule_Check(module)) {
         dict = PyModule_GetDict(module);