Disallow use of `ClassVar` for TypedDict and NamedTuple attributes

[erictraut]

The runtime disallows the use of the ClassVar qualifier when defining a TypedDict and NamedTuple.

This doesn't strike me as a significant source of confusion or bugs, but it has been proposed to amend the typing spec to mandate that type checkers report this condition.

[erictraut]

This is addressed in pyright 1.1.390.