type: use ClassVar decorator for class attributes

CONTRIBUTING.md

@@ -80,7 +80,7 @@ Again, the most checks can be performed by `tox` command, though some additional
                 >>> clazz = Clazz()
         """
   
-        CLAZZ_ATTRIBUTE = 42
+        CLAZZ_ATTRIBUTE: ClassVar[int] = 42
   
         def __init__(self, *args: Any, **kwargs: Any) -> None:
             """
@@ -96,6 +96,7 @@ Again, the most checks can be performed by `tox` command, though some additional
 * Type annotations are the must, even for local functions. For the function argument `self` (for instance methods) and `cls` (for class methods) should not be annotated.
 * For collection types built-in classes must be used if possible (e.g. `dict` instead of `typing.Dict`, `tuple` instead of `typing.Tuple`). In case if built-in type is not available, but `collections.abc` provides interface, it must be used (e.g. `collections.abc.Awaitable` instead of `typing.Awaitable`, `collections.abc.Iterable` instead of `typing.Iterable`). For union classes, the bar operator (`|`) must be used (e.g. `float | int` instead of `typing.Union[float, int]`), which also includes `typing.Optional` (e.g. `str | None` instead of `Optional[str]`).
 * `classmethod` should (almost) always return `Self`. In case of mypy warning (e.g. if there is a branch in which function doesn't return the instance of `cls`) consider using `staticmethod` instead.
+* Class attributes must be decorated as `ClassVar[...]`.
 * Recommended order of function definitions in class:
 
     ```python