Prevent Python from treating _Callable an iterable

## Problem
Due to lack of `__iter__` implementation, Python is attempting to iterate over `_Callable` when possible. It's generally an undesired behavior, given the purposes of the `_Callable` class.

You can easily reproduce the issue:
```python
[*__import__("lambdas")._]
```

## Implementation

That should be a no-brainer to fix and write tests for.

```
class _Callable:
__iter__ = None
```