GeezerVison

The expression `x + y` is evaluated using `type(x).__add__`, which may
differ from `x.__add__` if the `__add__` attribute has been directly
assigned on the instance.

Demonstration:

```python
class C:
    def __add__(self, other):
        return "from class"

c = C()
print(c + c)  # prints "from class"

c.__add__ = lambda other: "from instance"
print(c.__add__(c))  # prints "from instance"
print(type(c).__add__(c, c))  # prints "from class"

print(c + c)  # prints "from class"!
```

wchargin-branch: dunder-op-invocation-docs
wchargin-source: 543786396ecba124a332357c771cfd6b20493abe