Make syntax of generic overload variants in messages close to PEP 695

[JukkaL]

We used a custom syntax for type variable bounds and restrictions. Use PEP 695 syntax instead (or at least something closer to PEP 695 syntax).

[JukkaL]

This conflicts with #17392. I will wait until #17392 is merged and will fix the conflicts here.

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

steam.py (https://github.com/Gobot1234/steam.py)
- steam/gateway.py:825: note:     def [MsgT <: Message] wait_for(self, type[MsgT], /, *, check: Callable[[MsgT], bool] = ...) -> Future[MsgT]
+ steam/gateway.py:825: note:     def [MsgT: Message] wait_for(self, type[MsgT], /, *, check: Callable[[MsgT], bool] = ...) -> Future[MsgT]
- steam/gateway.py:825: note:     def [ProtoMsgT <: ProtobufMessage] wait_for(self, type[ProtoMsgT], /, *, check: Callable[[ProtoMsgT], bool] = ...) -> Future[ProtoMsgT]
+ steam/gateway.py:825: note:     def [ProtoMsgT: ProtobufMessage] wait_for(self, type[ProtoMsgT], /, *, check: Callable[[ProtoMsgT], bool] = ...) -> Future[ProtoMsgT]

discord.py (https://github.com/Rapptz/discord.py)
- discord/ui/modal.py:152: note:          def [ClientT <: Client] on_error(self, Interaction[ClientT], Exception, /) -> Coroutine[Any, Any, None]
+ discord/ui/modal.py:152: note:          def [ClientT: Client] on_error(self, Interaction[ClientT], Exception, /) -> Coroutine[Any, Any, None]
- discord/ext/commands/hybrid.py:814: note:          def [CommandT <: Command[Any, [VarArg(Any), KwArg(Any)], Any]] command(self, name: str = ..., cls: type[CommandT] = ..., *args: Any, **kwargs: Any) -> Callable[[Callable[[CogT, ContextT, **P], Coroutine[Any, Any, T]] | Callable[[ContextT, **P], Coroutine[Any, Any, T]]], CommandT]
+ discord/ext/commands/hybrid.py:814: note:          def [CommandT: Command[Any, [VarArg(Any), KwArg(Any)], Any]] command(self, name: str = ..., cls: type[CommandT] = ..., *args: Any, **kwargs: Any) -> Callable[[Callable[[CogT, ContextT, **P], Coroutine[Any, Any, T]] | Callable[[ContextT, **P], Coroutine[Any, Any, T]]], CommandT]
- discord/ext/commands/hybrid.py:838: note:          def [GroupT <: Group[Any, [VarArg(Any), KwArg(Any)], Any]] group(self, name: str = ..., cls: type[GroupT] = ..., *args: Any, **kwargs: Any) -> Callable[[Callable[[CogT, ContextT, **P], Coroutine[Any, Any, T]] | Callable[[ContextT, **P], Coroutine[Any, Any, T]]], GroupT]
+ discord/ext/commands/hybrid.py:838: note:          def [GroupT: Group[Any, [VarArg(Any), KwArg(Any)], Any]] group(self, name: str = ..., cls: type[GroupT] = ..., *args: Any, **kwargs: Any) -> Callable[[Callable[[CogT, ContextT, **P], Coroutine[Any, Any, T]] | Callable[[ContextT, **P], Coroutine[Any, Any, T]]], GroupT]

pydantic (https://github.com/samuelcolvin/pydantic)
- pydantic/v1/generics.py:124: note:     def [Model <: BaseModel] create_model(str, /, *, __config__: type[BaseConfig] | None = ..., __base__: type[Model] | tuple[type[Model], ...], __module__: str = ..., __validators__: dict[str, classmethod[Any, Any, Any]] = ..., __cls_kwargs__: dict[str, Any] = ..., **field_definitions: Any) -> type[Model]
+ pydantic/v1/generics.py:124: note:     def [Model: BaseModel] create_model(str, /, *, __config__: type[BaseConfig] | None = ..., __base__: type[Model] | tuple[type[Model], ...], __module__: str = ..., __validators__: dict[str, classmethod[Any, Any, Any]] = ..., __cls_kwargs__: dict[str, Any] = ..., **field_definitions: Any) -> type[Model]

[davidhalter]

@JukkaL Just FYI: I think it's a bit confusing that TypeStrVisitor.visit_callable_type still uses the old syntax (this causes old syntax in all reveal_type() calls). I have unfortunately no time to fix it now.