Fix pickling of exceptions with kw_only attributes

[worksbyfriday]

Summary

Fixes the TypeError that occurs when pickling attrs exception classes that use kw_only=True. BaseException.__reduce__ passes all attribute values as positional args, which fails when some attrs are keyword-only.

The fix adds a custom __reduce__ to all attrs exception classes (auto_exc=True) that uses a helper function _reconstruct_exc to pass all init arguments as keyword arguments. This handles:

• All-kw_only exceptions (kw_only=True on the class)
• Mixed positional + kw_only exceptions
• Subclass hierarchies with mixed kw_only inheritance
• Both slotted and non-slotted exceptions

Fixes #734

Test plan

• Added module-level test exception classes (slotted and non-slotted variants)
• test_auto_exc_kw_only_pickle: verifies pickle roundtrip for all-kw_only exceptions
• test_auto_exc_mixed_kw_only_pickle: verifies pickle roundtrip for mixed positional + kw_only exceptions
• All 1365 existing tests continue to pass

[hynek]

I'm sorry to be blunt, but this is AI slop. return cls(**kwargs) falls apart the moment private fields come into play. Use whatever tools you want, but use your judgement.

In case you want to give it another try, please note that we now have an AI policy.