uhd: Add uhd.find to the pybind11 interface

`uhd_find_devices` would be the default shell function to discover available USRPs.
PyUHD provides a function `uhd.find("")` that does the job. Besides, in
`gr-uhd/python/uhd/__init__.py` there is a definition of `find_devices`.
However, this function is broken at the moment because it tries to use a `find_devices_raw`
function from the gr-uhd pybind11 interface that is not available.
With this commit, we may be able to fix `find_devices` as well.

The actual change is just copied from UHD `host/lib/device_python.cpp`.
Alternative solutions would include a new function in C++ and a pybind11 wrapper.

Signed-off-by: Johannes Demel <demel@ant.uni-bremen.de>

gr-uhd/python/uhd/bindings/usrp_block_python.cc

@@ -25,6 +25,8 @@ void bind_usrp_block(py::module& m)
     m.attr("ALL_MBOARDS") = py::int_(::uhd::usrp::multi_usrp::ALL_MBOARDS);
     m.attr("ALL_LOS") = py::str(::uhd::usrp::multi_usrp::ALL_LOS);
 
+    m.def("find", [](const uhd::device_addr_t& hint) { return uhd::device::find(hint); });
+
     py::class_<usrp_block,
                gr::sync_block,
                gr::block,