Use __new__() only for initialization

[agronholm]

Fixes #46.

[coveralls]

Pull Request Test Coverage Report for Build 3757240936

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 64 unchanged lines in 6 files lost coverage.
• Overall coverage decreased (-0.03%) to 65.463%

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Files with Coverage Reduction	New Missed Lines	%
/opt/hostedtoolcache/Python/3.10.9/x64/lib/python3.10/site-packages/exceptiongroup/_exceptions.py	5	96.38%
/opt/hostedtoolcache/Python/3.7.15/x64/lib/python3.7/site-packages/exceptiongroup/_exceptions.py	5	96.21%
/opt/hostedtoolcache/Python/3.8.15/x64/lib/python3.8/site-packages/exceptiongroup/_exceptions.py	5	96.38%
/opt/hostedtoolcache/Python/3.9.16/x64/lib/python3.9/site-packages/exceptiongroup/_exceptions.py	5	96.38%
/opt/hostedtoolcache/PyPy/3.8.15/x64/lib/pypy3.8/site-packages/exceptiongroup/_exceptions.py	11	89.86%
/opt/hostedtoolcache/Python/3.11.1/x64/lib/python3.11/site-packages/exceptiongroup/_exceptions.py	33	0%

Totals
Change from base Build 3757221566:	-0.03%
Covered Lines:	2049
Relevant Lines:	3130

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💛 - Coveralls

[Zac-HD]

I haven't tested this locally, but from checking against CPython it looks good to me 👍

[agronholm]

Thanks! I'll cut a release later unless @KevinMGranger has objections.

[agronholm]

I initially meant to make this into a major release but on further inspection it looks like it might not need a major bump after all. Trio at least passes its test suite against this new version without any changes, and that's the most demanding downstream project I know of, so I'll make a minor release instead.