Support buffers in IO[bytes].write

[dvarrazzo]

Trying to annotate psycopg code in order to pass mypy check with disabled bytearray/memoryview promotion to bytes see branch mypy-0990.

Everything was smooth, except one questionable point:

from typing import IO, Union
from typing_extensions import TypeAlias

Buffer: TypeAlias = Union[bytes, bytearray, memoryview]

def write1(f: IO[bytes], data: Buffer) -> None:
    f.write(data)

Mypy 0.990 reports:

error: Argument 1 to "write" of "IO" has incompatible type "Union[bytes, bytearray, memoryview]"; expected "bytes"  [arg-type]

Same using BinaryIO instead of IO[bytes].

As far as I can see, a binary file supports any object supporting the buffer protocol (or maybe less than that? However, the three types in the union tested are definitely supported).

Declaring f: IO[Buffer] almost works: write() doesn't complain anymore, but IO does: Invalid type argument value for "IO".

Maybe something could be relaxed here?

[JelleZijlstra]

IO.write is declared as def write(self, __s: AnyStr) -> int: ..., so it only accepts exactly bytes if self is an IO[str]. Concrete binary IO classes do support any ReadableBuffer in their write methods, so it makes sense for IO[str] to do too. We can achieve that with overloads on self, similar to how we have handled re.Pattern.

The IO class is problematic because it doesn't exactly represent concrete IO classes well, and typeshed's recommendation is generally to use more specific protocols. However, IO is still widely used, so I think we should fix it. I'll submit a PR.