Fixed traceback exception only showing 2 nested levels of context/cause

[agronholm]

Fixes #36.

[coveralls]

Pull Request Test Coverage Report for Build 3392559033

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 208 unchanged lines in 6 files lost coverage.
• Overall coverage increased (+0.1%) to 74.622%

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Files with Coverage Reduction	New Missed Lines	%
/opt/hostedtoolcache/Python/3.10.8/x64/lib/python3.10/site-packages/exceptiongroup/_formatting.py	11	93.46%
/opt/hostedtoolcache/Python/3.7.15/x64/lib/python3.7/site-packages/exceptiongroup/_formatting.py	14	91.51%
/opt/hostedtoolcache/Python/3.8.14/x64/lib/python3.8/site-packages/exceptiongroup/_formatting.py	14	91.59%
/opt/hostedtoolcache/Python/3.9.15/x64/lib/python3.9/site-packages/exceptiongroup/_formatting.py	14	91.59%
/opt/hostedtoolcache/PyPy/3.8.13/x64/lib/pypy3.8/site-packages/exceptiongroup/_formatting.py	73	21.03%
/opt/hostedtoolcache/Python/3.11.0/x64/lib/python3.11/site-packages/exceptiongroup/_formatting.py	82	0%

Totals
Change from base Build 3390034820:	0.1%
Covered Lines:	1873
Relevant Lines:	2510

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💛 - Coveralls

[proto3]

Looks good to me, although I'm not a traceback module expert ^^'
It fixes the issue from #36, thanks !