Match filter attributes by identity

[Sean-Kenneth-Doherty]

Summary

Fixes #864 by making attrs.filters.include() and attrs.filters.exclude() match attrs.Attribute objects by identity instead of equality.

Root Cause

attrs.Attribute equality does not carry owning-class identity, so fields with the same name and otherwise equal metadata on different classes compare equal. Passing attrs.fields(SomeClass).field to a filter could therefore include or exclude same-named fields from unrelated classes during recursive asdict() conversion.

Changes

• Preserve Attribute filter arguments as an ordered tuple instead of a frozenset.
• Match Attribute arguments with is identity checks.
• Keep string-name filtering as the explicit way to match fields by name across classes.
• Add regression tests for both include() and exclude().
• Update the examples docs and add a changelog fragment.

Validation

• .venv/bin/python -m pytest -q tests/test_filters.py -> 35 passed
• .venv/bin/python -m pytest -q tests/test_filters.py -k "identity or split_what" -> 2 passed, 33 deselected
• .venv/bin/python -m pytest -q -> 1387 passed, 5 skipped, 1 xfailed
• .venv/bin/ruff check src/attr/filters.py tests/test_filters.py docs/examples.md
• .venv/bin/ruff format --check src/attr/filters.py tests/test_filters.py
• .venv/bin/python -m compileall -q src/attr/filters.py tests/test_filters.py
• git diff --check origin/main...HEAD