ModuleNotFoundError: No module named 'rdflib.Graph'

Hello. I'm currently getting;

```
python portmeta.py
Traceback (most recent call last):
  File "portmeta.py", line 3, in <module>
    from rdflib.Graph import Graph
ModuleNotFoundError: No module named 'rdflib.Graph'
make: *** [Makefile:37: gen/FluteMeta.h] Error 1
```

Though I have extra/python-rdflib installed.

Is this a it's-not-python3-ready problem?