Revert "Remove redundant keys() since dict is an iterator of keys"

This reverts commit 1ef0b85ff5f81ce20dcfce3d1c810dae55aeb1f7.

This caused a bunch of RuntimeError exceptions like the following:

```
Exception RuntimeError: 'dictionary changed size during iteration' in
<bound method SREQ.__del__ of <salt.payload.SREQ object at 0x7fde1c240590>> ignored
```

@jacksontj FYI