Specialising LOAD_ATTR for mutable class attributes

Currently the following code doesn't specialise:
```
from enum import Enum
class Color(Enum):
    RED = 1
    GREEN = 2
    BLUE = 3

def f():
 for _ in range(10):
  Color.RED

f()
```

This use pattern is very common in enums and also the recommended way to check constants in pattern matching. Specialising for ~this~ all mutable class attributes should speed up those use cases.
