Add `--warn-needless-override` option to `stubtest`

Feature

Imagine this file with implementation:

class A:
   def do_some(self) -> None:
       print('A')

class B(A):
   def do_some(self) -> None:
       print('B')

Auto-stub creators will create a stub like:

class A:
   def do_some(self) -> None: ...

class B(A):
   def do_some(self) -> None: ...

Which technically is right, but not quite.
Since do_some has the same signature in both A and B (only runtime implementation is different), we don't actually need it to be duplicated. We should ideally want just:

class A:
   def do_some(self) -> None: ...

class B(A): ...

Because we always want minimal correct stubs.
But, right now we only do this by hand.

Pitch

I propose adding --warn-needless-override options (with whatever name) that can warn us about needless overrides of parent methods in child classes.

I will send a PR with the initial imlementation.