Upgrade Enum str Mixin to StrEnum in Python 3.11

Given that Python 3.11 [introduced](https://blog.pecar.me/python-enum) breaking change for Enum with str/int Mixin wouldn't it make sense to auto-upgrade this to `StrEnum/IntEnum`? For example `pyupgrade --py311-plus`: 

Input:
```
class MyEnum(str, enum.Enum):
    FOO = "foo"
    BAR = "bar"
```

Output:
```
class MyEnum(enum.StrEnum):
    FOO = "foo"
    BAR = "bar"
```

