Patch sys.unraisablehook to format exception groups

[agronholm]

Whether this should be merged or not depends on the resolution of upstream issue (python/cpython#95572).

Fixes #18.

[coveralls]

Pull Request Test Coverage Report for Build 2884220953

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 21 unchanged lines in 3 files lost coverage.
• Overall coverage decreased (-0.2%) to 74.399%

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Files with Coverage Reduction	New Missed Lines	%
/opt/hostedtoolcache/PyPy/3.8.13/x64/lib/pypy3.8/site-packages/exceptiongroup/_formatting.py	4	21.52%
/opt/hostedtoolcache/Python/3.7.13/x64/lib/python3.7/site-packages/exceptiongroup/_formatting.py	5	82.91%
/opt/hostedtoolcache/Python/3.11.0-rc.1/x64/lib/python3.11/site-packages/exceptiongroup/_formatting.py	12	0%

Totals
Change from base Build 2862927764:	-0.2%
Covered Lines:	1610
Relevant Lines:	2164

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💛 - Coveralls