Fix attrs.frozen with enum.Enum subclasses

[C1-BA-B1-F3]

Description

This PR fixes issue #1287 where attrs.frozen classes don't work with enum.Enum subclasses.

Problem

When creating an Enum subclass of a frozen attrs class, the frozen __setattr__ would prevent Enum from creating members, raising a FrozenInstanceError:

@attrs.frozen
class AttrsFrozen:
    a: int
    b: float

# This would raise FrozenInstanceError
class AttrsFrozenEnum(AttrsFrozen, Enum):
    X = (1, 2.0)
    Y = (3, 4.0)

Solution

Modified _frozen_setattrs to allow Enum instances to bypass the frozen check. Enum needs to set attributes like _name_, _value_, etc. during member creation, so we allow this by using object.__setattr__ for Enum instances.

Changes

1. src/attr/_make.py: Added check in _frozen_setattrs to allow Enum instances to set attributes
2. tests/test_functional.py: Added test cases for Enum + attrs interaction
3. changelog.d/1287.change.md: Added changelog entry

Testing

• Added tests for both attrs.define and attrs.frozen with Enum subclasses
• Verified that frozen Enum subclasses are hashable
• All existing tests pass