gr_modtool bind makes incorrect file

[Jameson Collins]

I am using gnuradio 3.10.2 from conda.  If I make a new module and simply
edit its _impl.cc and yaml files then everything works correctly.

If I take the same module, and run `gr_modtool bind` on it then the new
binding file will be different and I'll get errors when trying to run.

Here is the code to start with:
```
    py::class_<newmod2, gr::sync_block, gr::block, gr::basic_block,
        std::shared_ptr<newmod2>>(m, "newmod2", D(newmod2))

        .def(py::init(&newmod2::make),
           D(newmod2,make)
        )
```

Here is the code after running `gr_modtool bind`:
```
    py::class_<newmod2,
        std::shared_ptr<newmod2>>(m, "newmod2", D(newmod2))

        .def(py::init(&newmod2::make),
           py::arg("myarg") =  false,
           D(newmod2,make)
        )
```