Operations on `None` should fall back to `object`

```
rwbarton@morphism:~/mypy$ python3 -m mypy --strict-optional -c 'None == None'
<string>:1: error: Unsupported left operand type for == (None)
rwbarton@morphism:~/mypy$ python3 -m mypy --strict-optional -c 'str(None)'
<string>:1: error: No overload variant of "str" matches argument types [builtins.None]
```

These should not be errors, as the type of `None` is a subtype of `object`.
