Proposal: add `@typing.format_specifiers`

This decorator factory would tell type checkers what format codes are supported by instances of that type. It would take variable positional arguments, each a string corresponding to a valid specification, and type checkers will know the class its returned decorator is applied to will only take those format specifications, so that it can flag mistyped format specifications and make subtle bugs clear. It, however, requires special-casing string formatting and the format builtin.

Currently it is possible to restrict the type of format_spec in __format__ to Literal[...], but that will cause Liskov substitution principle violations and type checkers may not consider it.

I am opening an issue here in hopes of getting more feedback because my DPO thread has gained little traction.

首先阅读提案及其中链接的 DPO 讨论串，因为尚未确定任何实现文件或测试。下一步是就 decorator 的行为和类型检查器的处理方式达成一致；完成的标准是有一份已接受的设计，其中定义了所需的特殊情况。