ByteString, specifying length for any bytes-like variable

This request is a combination of two additions to ByteString that apparently are not supported yet, or at least I could find no discussion of them.

• Support any bytes-like buffers, not only bytes, bytearray and memoryview
• Allow specifying the length of the buffer expected (in my case it usually must be exactly 32 bytes)

Ideally, Python typing would allow specifying bytes-like of (a) any length, (b) fixed length or (c) min and/or max length. Potentially also separately specifying if a writable buffer is expected.

While I don't expect for mypy to be able to check for all of that, it would at least be nice to include such information in the annotations for documentation purposes, avoiding the use of docstrings for parameter definitions where names and types are descriptive enough. Is there any way to do this without causing mypy errors, considering that I would rather have no type checking but documentative annotations than what the current implementation can do?

未指定文件、测试或入口点。首先检查现有的 ByteString 类型标注行为及相关的类型标注讨论，然后确定是否可以在不产生 mypy 错误的情况下表示类似 bytes 的类型、固定长度或有界长度以及可写性。完成的标准是达成一致的标注设计，并记录类型检查行为。