"typing.BinaryIO" missing "readinto" method

Bug report

Bug description:

typeing.BinaryIO is completely missing the readinto method. All binary mode return values of open support the readinto method, and the lack of it on the BinaryIO type makes type-hinting difficult when using this method. One could try to use io.RawIOBase | io.BufferedIOBase but this looses the fact that the result of open has a name property. This has been brought up in python/typing#659 and python/typeshed#2166.

Furthmore, the write method in BinaryIO is declared to only be compatible with Union[bytes, bytearray] when it should be anything that satisfies collections.abc.Buffer.

I propose that BinaryIO should look something more like:

class BinaryIO(IO[bytes]):
    """Typed version of the return of open() in binary mode."""

    __slots__ = ()

    @abstractmethod
    def readinto(self, b: collections.abc.Buffer) -> int:
        pass

    @abstractmethod
    def write(self, s: collections.abc.Buffer) -> int:
        pass

    @abstractmethod
    def __enter__(self) -> 'BinaryIO':
        pass

CPython versions tested on:

CPython main branch

Operating systems tested on:

No response

Start at the typing.BinaryIO definition and compare it with the binary-mode return values of open(), focusing on the proposed readinto and Buffer-compatible write signatures. Verify that BinaryIO still exposes the name property and that the updated typing behavior is covered by the relevant existing tests; the issue does not name a specific file or test.