python: Allow SubdevSpec to be implied from a string

This allows the following:

```py
my_usrp.set_rx_subdev_spec("A:0 B:0")
```

Without this change, only the following was possible:

```py
my_usrp.set_rx_subdev_spec(uhd.usrp.SubdevSpec("A:0 B:0"))
```

Because the implicit-from-string version is possible in C++, we now also
enable it in Python.

host/lib/usrp/subdev_spec_python.hpp

@@ -31,6 +31,8 @@ void export_subdev_spec(py::module& m)
             [](subdev_spec_t& self, const size_t idx) { return self.at(idx); })
 
         ;
+
+    py::implicitly_convertible<std::string, uhd::usrp::subdev_spec_t>();
 }
 
 #endif /* INCLUDED_UHD_USRP_SUBDEV_SPEC_PYTHON_HPP */