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.

Start by reading the proposal and its linked DPO thread, since no implementation files or tests are identified. The next step is to establish agreement on the decorator's behavior and type-checker handling; done would mean an accepted design with the required special cases defined.