Fix bridgedb.test.deprecated.is_valid_ip() doctest.

Even the doctests in `bridgedb.test.deprecated` get run! Since this
deprecated function no longer lives in the `bridgedb.Bridges` module, we
can't import it from there in its doctest.

 * CHANGE doctest in `bridgedb.test.deprecated.is_valid_ip()` docstring
   to import itself from the `bridgedb.test.deprecated` module.