Pyright should narrow except* to ExceptionGroup with non-base exceptions

[erictraut]

Discussed in #9455

^{Originally posted by jakkdl November 13, 2024}
It was changed from ExceptionGroup to BaseExceptionGroup in microsoft/pylance-release#4183
Related typeshed issue python/typeshed#12972

I'm not 100% if the typeshed issue will resolve except* handling in pyright, in case it would then we can directly resolve/close this. But in case it's hard-coded to always be BaseExceptionGroup and won't be changed by typeshed messing with __new__ then you might want to fix it right away.

repro:

from typing import reveal_type
try:
    raise ExceptionGroup("", [ValueError()])
except* ValueError as e:
    reveal_type(e)

$ python repro.py
Runtime type is 'ExceptionGroup'
$ pyright repro.py
  repro.py:5:17 - information: Type of "e" is "BaseExceptionGroup[ValueError]"
0 errors, 0 warnings, 1 information 
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