Match filter attributes by identity instead of equality

[PranavMishra28]

What was broken

attrs.filters.include / exclude accept types, field-name strings, and Attribute instances. The Attribute instances were stored in a frozenset and matched with attribute in attrs, i.e. by equality. But Attribute.__eq__/__hash__ ignore the owning class, so two identically-configured same-named attributes on different classes compare equal:

@attr.s
class Robber:
    repeated = attr.ib(default=1)
    only_robber = attr.ib(default=2)

@attr.s
class Cop:
    repeated = attr.ib(default=1)   # identically configured
    only_cop = attr.ib(default=4)

attr.asdict(Cop(), filter=attr.filters.exclude(attr.fields(Robber).repeated))
# -> {'only_cop': 4}   # Cop.repeated wrongly dropped

Excluding (or including) one class's attribute silently affects an identically-configured attribute of an unrelated class.

Why it matters

This is the behavior tracked in #864, which is labeled Bug. As noted there, Attribute instances are independent of the classes they're defined on, so equality-based matching "is absolutely not what people would expect." attr.fields(A).x and attr.fields(B).x are distinct objects, so identity is the correct test.

What the fix does

_split_what keeps the passed attributes in a tuple (instead of a frozenset), and include/exclude match them with any(attribute is a for a in attrs) — by identity. Since fields() returns the same cached Attribute object for a given class, the intended attribute still matches, while an equal-but-distinct attribute on another class no longer does.

What it deliberately does not change

• Type and field-name-string matching are untouched. The cross-class "drop this field name from many classes" use case raised in asdict filter exclude list matches on name instead of attribute #864 is already served by passing the name string (exclude("repeated")), which matches by name across classes.
• Public include/exclude signatures and the .pyi stub are unchanged.

Note on compatibility

This narrows Attribute matching from equality to identity. Anyone who (perhaps unknowingly) relied on the equal-but-distinct cross-class match would see a behavior change; per the issue this was considered a bug rather than intended behavior, so I filed it under change. Happy to reclassify as breaking if you'd prefer.

Testing

• New identity regression tests for both include and exclude (equal-but-distinct attribute on another class is not matched); updated the _split_what test for the tuple.
• Full suite green: 1383 passed, 9 skipped, 1 xfailed; ruff check and ruff format --check clean. Added changelog.d/864.change.md.

Fixes #864

Developed with Claude Code; reviewed and tested by Pranav before marking ready for review.

[PranavMishra28]

just wanted to highlight that this is ready for a look whenever you have a moment. It directly fixes the equality bug in #864 and all 19 checks are passing green.