fix(slots): preserve __dict__ state when pickling slotted classes

[saadkhantareen]

Summary

A slotted attrs class that inherits from a non-slotted class has an instance __dict__, but the generated __getstate__ / __setstate__ only handled the attrs fields.
Any additional state stored in that __dict__ -- for example an attribute set in __attrs_post_init__ or by a mixin -- was silently dropped when the instance was pickled (which also breaks multiprocessing, as reported in #1020) or copied.

This change makes the generated state handling preserve that __dict__ state, mirroring what object.__getstate__() returns for such classes, while keeping the existing state format.

Changes

• src/attr/_make.py: in _ClassBuilder._make_getstate_setstate(), slots_getstate now merges entries from the instance __dict__ that are not attrs fields into the returned state dict (attrs fields take precedence). slots_setstate restores those extra entries into the instance __dict__ when the instance has one. Instances without a __dict__ ignore extra keys as before, so unpickling after an attribute was removed from a slotted class keeps working, and both the pre-22.2.0 tuple state format and the current dict format remain readable.
• tests/test_slots.py: added a slotted subclass of a non-slotted attrs class whose __attrs_post_init__ stores an extra attribute in __dict__, with regression tests covering a pickle round-trip for every protocol and copy.copy.
• changelog.d/1020.change.md: added a news fragment.

Tests

• PYTHONPATH=src python -m pytest tests/test_slots.py -q -- 59 passed, 1 skipped
• PYTHONPATH=src python -m pytest tests/test_dunders.py tests/test_funcs.py -q -- 152 passed
• ruff format --check src/attr/_make.py tests/test_slots.py and ruff check src/attr/_make.py tests/test_slots.py -- clean

Fixes #1020

Pull Request Checklist

• I acknowledge this project's AI policy.
• This pull request is not from my main branch.
• There's tests for all new and changed code.
• Changes or additions to public APIs are reflected in our type stubs (files ending in .pyi).
  • ...and used in the stub test file typing_tests/baseline.py or, if necessary, typing_tests/mypy.py.
  • If they've been added to attr/__init__.pyi, they've also been re-imported in attrs/__init__.pyi.
• The documentation has been updated.
  • New functions/classes have to be added to docs/api.rst by hand.
  • Changes to the signatures of @attr.s() and @attrs.define() have to be added by hand too.
  • Changed/added classes/methods/functions have appropriate versionadded, versionchanged, or deprecated directives.
  • Documentation in .rst and .md files is written using semantic newlines.
• Changes have news fragments in changelog.d.

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