graph: Misleading reporting of node in cycle for CycleError

Initially reported by @majosm.

With this code:

```python
graph={1: {}, 5: {1, 8}, 8: {5}}

from pytools.graph import compute_topological_order
compute_topological_order(graph)
```


`compute_topological_order` reports:

```
File "/Users/mdiener/Work/emirge/pytools/pytools/graph.py", line 303, in compute_topological_order
raise CycleError(next(iter(n for n, num_preds in
pytools.graph.CycleError: 1
```