Updated type annotations to match typeshed

[agronholm]

Fixes #76.

[jakkdl]

I downloaded stdlib/builtins.pyi and pruned+reformatted both it and exceptions/_exceptions.py to only have the type stubs remaining and ran a diff on them - here's the result:

only in typeshed

def __init__(
    self, __message: str, __exceptions: Sequence[_BaseExceptionT_co]
) -> None: ...

def __init__(
    self, __message: str, __exceptions: Sequence[_ExceptionT_co]
) -> None: ...

def __class_getitem__(cls, __item: Any) -> GenericAlias: ...

only in exceptiongroup/_exceptions.py

 def add_note(self, note: str) -> None: ...

def __str__(self) -> str: ...
def __repr__(self) -> str: ...

I suspect a couple of the above are intentional / does not matter.

there's otherwise no difference of signatures for functions which is great, assuming the types in typeshed are correct it means this PR is aok.

[agronholm]

Thanks for reviewing!