pybind11 missing symbols in rule

[manthey]

After the pybind11 change, in src/mapnik_rule.cpp, the symbols (and copy_symbols) definitions aren't present. This means that the sample code at https://github.com/mapnik/mapnik/wiki/GettingStartedInPython#create-a-style will fail with AttributeError: 'mapnik._mapnik.Rule' object has no attribute 'symbols'