Fix walk_trace_chain: change container for seen_exceptions

[Safintim]

Description of the change

What if, for some reason, the exception is not hashable?

I ran into this problem using the library https://github.com/ecederstrand/exchangelib
Here's my pr ecederstrand/exchangelib#1097

I don't know why the author decided to do that. Asked the author, haven't received an answer yet. Anyway, it seems to me that it's worth providing for such a possibility.

Type of change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Maintenance
• New release

Checklists

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