TracebackException holds a reference to the exc_traceback, which is wrong because (1) it's supposed to capture the output without holding references to real things. (2) it makes comparison wrong for equivalent exceptions, as in this example:

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import sys
import traceback

excs = []
for _ in range(2):
    try:
        1/0
    except:
        excs.append(traceback.TracebackException(*sys.exc_info()))

print('formats equal: ', list(excs[0].format()) == list(excs[0].format()))
print('excs equal: ', excs[0] == excs[1])
excs[0].exc_traceback = excs[1].exc_traceback = None
print('excs equal w/o exc_traceback: ', excs[0] == excs[1])

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Output:

formats equal:  True
excs equal:  False
excs equal w/o exc_traceback:  True

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The good news is that it's only used to check for non-None (added here https://bugs.python.org/issue24695) so should be easy to remove.