Removed continued parametrization of FilePathOrBuffer
follow up to #27598 to simplify things a bit. I think it was a mistake to add `IO[AnyStr]` as `AnyStr` is a TypeVar and not something to actually parametrize with. The source for `IO` already does this:

https://github.com/python/cpython/blob/8990ac0ab0398bfb9c62031288030fe7c630c2c7/Lib/typing.py#L1452

 I'm not sure why mypy doesn't error on this. 

@simonjayhawkins 