Mypy fails when exception handler is turned on

[gcarvellas]

Describe the bug
mypy fails because of the exception handler in app.py. It's commented out right now but it shouldn't be.

[Yun-L]

the error handler decorator expects a function that returns a ResponseReturnValue, defined in https://github.com/pallets/flask/blob/ed5b240417414cbd0322efa95c91f759928ba154/src/flask/typing.py#L32

our FlaskResponseType gives us a type error, but looks like it's almost the same as ResponseValue (a subtype of ResponseReturnValue. Is there a reason why we are using our own return type for route functions instead of the flask defined one?

[gcarvellas]

the error handler decorator expects a function that returns a ResponseReturnValue, defined in https://github.com/pallets/flask/blob/ed5b240417414cbd0322efa95c91f759928ba154/src/flask/typing.py#L32

our FlaskResponseType gives us a type error, but looks like it's almost the same as ResponseValue (a subtype of ResponseReturnValue. Is there a reason why we are using our own return type for route functions instead of the flask defined one?

Let's use the flask defined return types instead