Disallow ClassVar and Final qualifiers in TypedDict and NamedTuple items

[brianschubert]

Both ClassVar and Final produce runtime errors when used in the body of a TypedDict or NamedTuple.

Mypy already emits an error for Final, but the error message could be clarified.

class TD(TypedDict):
    a: Final[int]     # E: Final can be only used as an outermost qualifier in a variable annotation
    b: ClassVar[int]  # Runtime error, no mypy error

class NT(NamedTuple):
    a: Final[int]     # E: Final name must be initialized with a value \
                      # E: Final can be only used as an outermost qualifier in a variable annotation
    b: ClassVar[int]  # Runtime error, no mypy error

Follows from

• Typing spec update: ClassVar and Final within TypedDict typing-council#39

• Disallow use of ClassVar for TypedDict and NamedTuple attributes microsoft/pyright#9526