Add Type Hints to `FlaskView` Class Properties that are Typically Strings but Initialized as None

[dcloutman]

I have noticed that when I override the value of a class property that is defined in FlaskView, my IDE (VSCode) complains about incomparable types. Specifically, this occurs when I assign a string value to FlaskView.base_route, and I presume the same issue would persist if I assigned a value similarly to FlaskView.route_prefix. I tried hacking the core library a bit, and adding the type hint "str|None" seemed to make the IDE complaint go away. Without the explicit type hint, the IDE assumes that the property's inferred type is None, which is not the case in practical usage.

[Flix6x]

I would definitely not object to adding type hints in a few places. Care to make a PR?

[dcloutman]

Sure. I'll make a fork and get to work on it.