make PytestReturnNotNoneWarning a RemovedIn8Warning by removing the `…

[graingert]

…@final` decorator

[bluetech]

The @final annotations are really just a way to say "we did not design this class for inheritance, if you inherit from it you're on your own". But it's a very weak thing, if we actually want to subclass some class then we should just remove the @final from it, i.e. I don't think the @sealed placeholder is necessary.

[nicoddemus]

Other than the existing comments we also need a new changelog entry. 👍

[nicoddemus]

Thanks @graingert!