Export DataclassInstance protocol from _typeshed

[cdce8p]

Add DataclassInstance protocol to _typeshed. Especially when dealing with mixin classes, it's quite common to have to use it. Moving it to _typeshed means users can import it from there without having to reimplement it themselves.

Followup to #9362 (comment)
/CC @AlexWaygood

[AlexWaygood]

Any reason not to just put it in _typeshed/__init__.pyi? It feels a bit small to go in its own file. We can always move it to a dataclasses-specific file later if we add more dataclasses-specific protocols or aliases.

Also: I'd like to add a comment noting that the protocol is quasi-experimental, and linking to the pyright issue. The type won't work as intended for pyright users unless and until the pyright issue is reconsidered (or we think of another way of doing this in the stubs).

[AlexWaygood]

Thanks!

[github-actions]

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

porcupine (https://github.com/Akuli/porcupine)
- porcupine/utils.py:408: note:     def asdict(obj: _DataclassInstance) -> Dict[str, Any]
+ porcupine/utils.py:408: note:     def asdict(obj: DataclassInstance) -> Dict[str, Any]
- porcupine/utils.py:408: note:     def [_T] asdict(obj: _DataclassInstance, *, dict_factory: Callable[[List[Tuple[str, Any]]], _T]) -> _T
+ porcupine/utils.py:408: note:     def [_T] asdict(obj: DataclassInstance, *, dict_factory: Callable[[List[Tuple[str, Any]]], _T]) -> _T

hydra-zen (https://github.com/mit-ll-responsible-ai/hydra-zen)
- src/hydra_zen/structured_configs/_implementations.py:1862: error: Incompatible types in assignment (expression has type "Union[Type[_DataclassInstance], Type[Any], Callable[P, R], Callable[..., Any], DataClass_]", variable has type overloaded function)  [assignment]
+ src/hydra_zen/structured_configs/_implementations.py:1862: error: Incompatible types in assignment (expression has type "Union[Type[DataclassInstance], Type[Any], Callable[P, R], Callable[..., Any], DataClass_]", variable has type overloaded function)  [assignment]

core (https://github.com/home-assistant/core)
- homeassistant/components/p1_monitor/diagnostics.py:45: error: Argument 1 to "asdict" has incompatible type "Optional[Any]"; expected "_DataclassInstance"  [arg-type]
+ homeassistant/components/p1_monitor/diagnostics.py:45: error: Argument 1 to "asdict" has incompatible type "Optional[Any]"; expected "DataclassInstance"  [arg-type]
- homeassistant/components/zwave_js/discovery.py:78: note:     def asdict(obj: _DataclassInstance) -> Dict[str, Any]
+ homeassistant/components/zwave_js/discovery.py:78: note:     def asdict(obj: DataclassInstance) -> Dict[str, Any]
- homeassistant/components/zwave_js/discovery.py:78: note:     def [_T] asdict(obj: _DataclassInstance, *, dict_factory: Callable[[List[Tuple[str, Any]]], _T]) -> _T
+ homeassistant/components/zwave_js/discovery.py:78: note:     def [_T] asdict(obj: DataclassInstance, *, dict_factory: Callable[[List[Tuple[str, Any]]], _T]) -> _T