Slicing with 0d ArraySymbol raises TypeError

Might be a misunderstanding on my part, but I'd expect the below to work:

```python
from dwave.optimization.mathematical import maximum
from dwave.optimization.model import Model

model = Model()
zero = model.constant(0)
one = model.constant(1)
data = model.constant([1, 1])
max = maximum(zero, one)
data[max:] # works fine if you omit the : here
```

File "dwave/optimization/symbols.pyx", line 604, in dwave.optimization.symbols.BasicIndexing.cppslice
TypeError: 'dwave.optimization.symbols.Maximum' object cannot be interpreted as an integer