fix: compare Attribute instances by identity in include/exclude

[TrueFurina]

Summary

Fixes #864: exclude(fields(A).foo) / include(fields(A).foo) matched Attribute instances by equality, so two classes sharing a same-named attribute (e.g. repeated_field_name) would both be affected, even though only one was passed to the filter.

Attribute instances are independent from the classes they are defined on, and == compares their contents — so a same-named attribute on another class compared equal. The fix compares by identity (is), matching exactly the attribute instance the caller passed.

Changes

• src/attr/filters.py: attribute in attrs → any(attribute is a for a in attrs) in both include_ and exclude_
• tests/test_filters.py: add TestSameNameAcrossClasses with C2/D2 sharing a repeated attribute, asserting exclusion/inclusion of one class's attribute does not affect the other

Verification

• Reproduced locally against attrs 26.1.0: before the fix, exclude(fields(C2).repeated) also rejected fields(D2).repeated (cross-class false positive)
• After the fix: exclude(fields(C2).repeated) rejects C2.repeated but allows D2.repeated; include behaves symmetrically
• ast.parse passes on both files; existing TestInclude/TestExclude parametrized cases are unaffected (single-class scenarios compare identically)