RT_UNARY_OP_NOT is implemented as if it was RT_UNARY_OP_BOOL

It seems that for almost (?) all scalar types it is implemented in a way like:

```
case RT_UNARY_OP_NOT: if (val != 0) { return mp_const_true;} else { return mp_const_false; }
```

With the effect of

```
>>> not 0
False
```

Python does not define **not** (http://docs.python.org/3.3/reference/datamodel.html#special-method-names), so we'd rather have RT_UNARY_OP_BOOL instead.
