Just a note, __context__ cycles can theoretically be longer than 2 nodes. I've encountered cycles like `exc.__context__.__context__.__context__ is exc` a few times in my life, typically resulting from some weird third-party libraries.

The only solution is to use a `set()` collection to track already visited exceptions.

To make it fast I propose to modify the code to:

1. Do a fast traverse with a regular while loop without tracking (no set())

2. If the number of iterations is longer than 100 and there's still no top context found -- go to (3)

3. Do a slower implementation with set()