BaseExceptionGroup should return ExceptionGroup if initialized with non-base exceptions

When initializing a BaseExceptionGroup with non-base exceptions the stdlib (and the backport) will in fact return an ExceptionGroup. The typing in neither of typeshed nor the backport currently supports this.

from typing_extensions import reveal_type

x = BaseExceptionGroup('', [ValueError()])
reveal_type(x)

$ python foo.py
Runtime type is 'ExceptionGroup'
$ mypy foo.py 
foo.py:30: note: Revealed type is "builtins.BaseExceptionGroup[builtins.ValueError]"
$ pyright foo.py 
foo.py
  foo.py:30:13 - information: Type of "x" is "BaseExceptionGroup[ValueError]"

I have vague recollections that trying to do this was hard-to-impossible, but I currently cannot find any related issues.

首先定位 typeshed 和 backport 中的 BaseExceptionGroup 定义，然后将它们的构造函数类型标注与 issue 中展示的运行时行为进行比较。使用 mypy 和 pyright 检查示例；当只传入非基础异常时，两个类型检查器都能推断出 ExceptionGroup[ValueError]，即表示完成。