py3.7 fix

PyUnicode_AsUTF8() returns const char* in py3.7 instead of char*. Making
changes to reflect that which should also be safe in
other Python versions.

src/Gui/WidgetFactory.cpp

@@ -1437,7 +1437,7 @@ Py::Object PyResource::setValue(const Py::Tuple& args)
 #endif
                 continue;
 #if PY_MAJOR_VERSION >= 3
-            char* pItem = PyUnicode_AsUTF8(item);
+            const char* pItem = PyUnicode_AsUTF8(item);
 #else
             char* pItem = PyString_AsString(item);
 #endif