Can't use Request / Response subclasses with set_error_serializer

[davetapley]

falcon/falcon/app.py

Line 960 in 6d7a45b

def set_error_serializer(self, serializer: ErrorSerializer) -> None:

falcon/falcon/_typing.py

Line 88 in 6d7a45b

ErrorSerializer = Callable[['Request', 'Response', 'HTTPError'], None]

Isn't quite right if you use request_type= and/or response_type= in App initializer.
The set_error_serializer implementation gets the subclasses at run time, but:

Argument of type "(request: Request, response: Response, error: HTTPError) -> None" cannot be assigned to parameter "serializer" of type "ErrorSerializer" in function "set_error_serializer"
  Type "(request: Request, response: Response, error: HTTPError) -> None" is not assignable to type "ErrorSerializer"
    Parameter 1: type "Request" is incompatible with type "Request"
      "falcon.request.Request" is not assignable to "my.Request"
    Parameter 2: type "Response" is incompatible with type "Response"
      "falcon.response.Response" is not assignable to "my.Response"

Pylance[reportArgumentType](https://github.com/microsoft/pyright/blob/main/docs/configuration.md#reportArgumentType)

---

I think an elegant solution would be to replace:

falcon/falcon/app.py

Lines 267 to 268 in 6d7a45b

	_request_type: Type[Request]
	_response_type: Type[Response]

With e.g.:

_REQ = TypeVar('_REQ', bound=Request)
_RES = TypeVar('_RES', bound=Response)

...

 _request_type: _REQ
 _response_type: _RES

...

    def set_error_serializer(self, serializer: ErrorSerializer[_REQ, _RES]) -> None:

Then:

falcon/falcon/_typing.py

Line 88 in 6d7a45b

ErrorSerializer = Callable[['Request', 'Response', 'HTTPError'], None]

Becomes:

ErrorSerializer = Callable[[_REQ, _RES, 'HTTPError'], None]

Thoughts?

[davetapley]

Whoops, dupe of #2372