Better message if override violates Liskov substitution principle

[JukkaL]

Some users don't understand why mypy complains about incompatible overrides. We could add an extra note in cases where the argument type is narrower in an override or the return type is wider (or all incompatible overrides). The note could mention Liskov substitution principle and add a link to mypy documentation (add a new section to https://mypy.readthedocs.io/en/latest/common_issues.html perhaps). We can also mention that # type: ignore[override] can be used to silence this error.

#7992 is an example where this came up, but several users have been confused about this.

[ilevkivskyi]

Another example is overriding writeable attribute with read-only, see #8185. We can probably just add a note to the standard error in such cases.

[rahulrj]

Can i submit an PR for this if no one else is working on this?

[JelleZijlstra]

@rahulrj please do, all contributions are appreciated!

[SwagatSBhuyan]

Is this issue still open? I see it has been merged as well, so I don't know if I should proceed here.