While working on several mypy plugins I had often step on the fact you cannot subclass any of the mypy classes, because they are compiled with mypyc. It caused runtime TypeError in the plugin's code.

But, when I was trying to run mypy on the plugin's code - it was fine, no error was shown. Because type are not @final and mypy is not able to understand that this is a problem.

So, I propose to add explicit @final annotations to almost all types, exception:

• Types with allow_interpreted_subclasses
• Types that are subclassed inside mypy itself: Node, Expression, TypeQuery, etc

One more strong point to making this change is that many classes are not "designed" to be subclassed.
So, this should not be a problem.

Related #10125
Related #9602