SupportsAbs not compatible with NamedTuple

[rwbarton]

from typing import SupportsAbs, NamedTuple

class Point(NamedTuple('Point', [('x', int), ('y', int)]), SupportsAbs[int]):
    def __abs__(p) -> int:
        return abs(p.x) + abs(p.y)

def test(a: Point) -> bool:
    return abs(a) == 2  # E: Argument 1 to "abs" has incompatible type "Point"; expected SupportsAbs[None]

There's no error in test if the NamedTuple superclass of Point is removed.