Optional strict checking of isinstance checks

[JukkaL]

The current semantics of isinstance checks make some assumptions about inheritance, such as there being no multiple inheritance from certain classes. This is probably reasonable as a default, since it makes easier to annotate legacy codebases. However, we could provide an option to have safer and stricter checking of isinstance checks. The exact behavior of this mode is still not quite clear.

See discussion in #3605 for further context.

[ilevkivskyi]

Just for the reference here is the previous proposal #3603 (comment)

[msullivan]

I don't think this meets the bar of adding a new flag and we're moving towards more intersection stuff