Fix pickling of exception classes with kw_only attributes (#734)

[r266-tech]

Problem

Exception classes decorated with auto_exc=True and kw_only=True cannot be pickled. BaseException.__reduce__ returns (cls, self.args, state), passing self.args as positional arguments to cls() on unpickle. When kw_only=True, __init__ only accepts keyword-only arguments, causing:

TypeError: __init__() takes 1 positional argument but 2 were given

Reproducer:

import attr, pickle

@attr.s(slots=False, auto_exc=True, kw_only=True)
class KwOnly(Exception):
    field: int = attr.ib()

pickle.loads(pickle.dumps(KwOnly(field=1)))  # TypeError

Fix

Added a custom __reduce__ for exception classes that have any kw_only attrs. It uses a _rebuild_exc helper that:

1. Creates the instance via cls.__new__(cls) — bypassing __init__
2. Restores attributes directly via object.__setattr__
3. Calls BaseException.__init__ to properly restore self.args

The fix is minimal (only applied when the class is an exception AND has kw_only attrs) and does not affect non-exception classes or exceptions without kw_only.

Tests

Added test_auto_exc_kw_only_pickle (parametrized over slots, frozen, pickle protocol) and test_auto_exc_mixed_kw_only_pickle (mixed positional + kw_only attrs).

Closes #734

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