chore: use Final type annotation on every top-level constant

[mayeut]

per #996 (comment), this annotates top-level constants as such.

[joerick]

Why this type is called Final and not Const baffles me. But either way, looks good!

[henryiii]

I think because it only refers to the reference assignment, not methods.

x: Final = [3]
x.append(4)

Is sadly completely valid. It protects against =, including +=, but does not understand "constness" of methods (which it can't, there's no const for methods in Python). It really is const only if the type is immutable.