Fixed AttributeError when rendering an excgroup as a cause

[agronholm]

This adopts Python 3.10 TracebackException semantics. I also changed the various patched exception rendering functions to pass compact=True like py3.10 does. It has no effect on older Pythons.

There's some duplication in the initialization of the TE but I couldn't figure out any other sane way to fix this.

Fixes #33. Closes #35.

[coveralls]

Pull Request Test Coverage Report for Build 3335397030

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 203 unchanged lines in 6 files lost coverage.
• Overall coverage decreased (-0.08%) to 74.524%

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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	15	90.4%
/opt/hostedtoolcache/Python/3.7.15/x64/lib/python3.7/site-packages/exceptiongroup/_formatting.py	15	89.8%
/opt/hostedtoolcache/Python/3.8.14/x64/lib/python3.8/site-packages/exceptiongroup/_formatting.py	15	89.9%
/opt/hostedtoolcache/Python/3.9.15/x64/lib/python3.9/site-packages/exceptiongroup/_formatting.py	15	89.9%
/opt/hostedtoolcache/PyPy/3.8.13/x64/lib/pypy3.8/site-packages/exceptiongroup/_formatting.py	67	23.23%
/opt/hostedtoolcache/Python/3.11.0/x64/lib/python3.11/site-packages/exceptiongroup/_formatting.py	76	0%

Totals
Change from base Build 3324050034:	-0.08%
Covered Lines:	1799
Relevant Lines:	2414

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💛 - Coveralls

[gschaffner]

Looks good to me! I also haven't been able to think of a saner way to do this than duplicating the recursion unroller from stdlib.

I think this also fixes a related bug #35. Should a test be added for this?

[agronholm]

Done and done.