generated bindings not compatible with Python 3.9

This is an upstream pybindgen problem; am noting it here so that people know it is being worked.

In some (not all) configurations on systems with Python 3.9, ns-3 will fail to compile the bindings, such as here:

src/antenna/bindings/ns3module.h:1200:27: error: 'PyEval_ThreadsInitialized' is deprecated [-Werror,-Wdeprecated-declarations]
        __py_gil_state = (PyEval_ThreadsInitialized() ? PyGILState_Ensure() : (PyGILState_STATE) 0);

Current workaround is to pass --disable-werror flag to the ./waf configure step.