rework class protocol check pattern

[altendky]

Purpose:

Current Behavior:

New Behavior:

Testing Notes:

[arvidn]

what's the behavior of moving those checks into the class body itself?

[altendky]

It makes them located closer to the class code where I suspect people would expect it to be. This should make the pattern more apparent to people (sorta...) and avoid stuff like #14991. In terms of functional change, you get a class var instead of a global (not a big deal) and the cast() pattern avoids 'instantiating' the class which makes it more flexible for use with classes that require parameters to be instantiated.

[xdustinface]

you get a class var instead of a global

Do we actually get the class var in there since it's only happening if TYPE_CHECKING?

[altendky]

If you're type checking you do. ;] Which may include IDEs, I'm not sure.