Two crashes on recursive named tuples

[ilevkivskyi]

While working on #3322 I have found two more crashes:

N = NamedTuple('N', [('x', N)])
n: N

crashes with RecursionError and

class N(NamedTuple):
 x: N
class M(NamedTuple):
 x: M

n: N
m: M
lst = [n, m]

crashes with AssertionError in join_instances_via_supertype. I didn't try TypedDict but most probably it will have similar problems.

There is actually a fundamental question here: what should be the join(N, M)? @JukkaL do you have any ideas? One possible option would be to detect recursive joins and just return object for them (maybe also UninhabitedType for recursive meets).

[JukkaL]

object would be consistent as the result of join(N, M). A recursive type like T where T = Tuple[T] would perhaps also be a reasonable result, assuming we were able to represent them. (The only reasonable common use case where recursive types could be useful that I've seen is JSON, so this is not a valid motivating example for recursive types.)