Warn on ExceptionGroup subclass without `derive`

[A5rocks]

While there's probably some usecase which makes sense for this, at most someone could just implement like so:

class CustomEG(ExceptionGroup):
  def derive(self, exs):
    return ExceptionGroup(self.message, exs)

to get the original behavior (I think). From python-trio/trio#3175

[jakkdl]

I was thinking that maybe this would fit better in flake8-bugbear due to being exceptiongroup-related and not necessarily async-specific, but given #302, #311, #301, #298 I think the best approach is creating a new 4xx series of rules for exceptiongroup-specific warnings.