Raise a TypeError when slice-setting a Multirange with a non-iterable value

Raise a TypeError when slice-setting a Multirange with a non-iterable value

The type declaration is:

    def __setitem__(index: slice, value: Iterable[Range[T]]) -> None:
        ...

so passing a non-iterable value should not be allowed. However, the
method implementation makes a special-case for non-iterable values with
a slice.

In accordance with MutableSequence behavior, e.g.:

  >>> x = [1, 2, 3]
  >>> x[1:3] = 0
  Traceback (most recent call last):
    File "<stdin>", line 1, in <module>
  TypeError: can only assign an iterable

it seems more correct to raise a TypeError.