The codecs module mostly but not fully supports non str<->bytes codecs

Bug report

Bug description:

There's in the codecs module an option for charbuffertype which allows bytes<->bytes codecs, but if there were a type for the encoded value, it would allow cleaner implementation of str<->str codecs, or more...
Most codecs in CPython use decoded str and encoded bytes, there are also codecs that have decoded bytes and encoded bytes, like base64 (note that a base64 codec arguably could be one that considers decoded bytes and encoded str).

I had been more or less perverting the codecs module to implement things like COBS (fundamentally bytes<->bytes), bubblebabble (I did bytes<->bytes but it should encode to str), and recently ANSI escape filtering, as a str<->str codec that works by implementing a virtual terminal, with a screen buffer. To make it work I only had to do self.bytebuffer = "" in the StreamReader constructor, but since there's a charbuffertype, maybe we could use a bytesbuffertype, and open the flood gates to all kinds of creative abuse.

CPython versions tested on:

3.12

Operating systems tested on:

Linux

まず codecs モジュールから始め、既存の charbuffertype の動作を含めて StreamReader コンストラクターを調査します。エンコード済みおよびデコード済みの値型について意図されたモデルを特定し、そのうえで str/bytes を超える、より広範な codecs 対応に何を含めるべきかを定義します。その範囲と期待される動作が確立され、実装されて初めて作業は完了です。