Stop changing return type annotations to tuples (#2384)

This fixes a bug where a trailing comma would be added to a
parenthesized return annotation changing its type to a tuple.
Here's one case where this bug shows up:

```
def spam() -> (
    this_is_a_long_type_annotation_which_should_NOT_get_a_trailing_comma
):
    pass
```

The root problem was that the type annotation was treated as if it was
a parameter & import list (is_body=True to linegen::bracket_split_build_line)
where a trailing comma is usually fine. Now there's another check in the
aforementioned function to make sure the body it's operating on isn't
a return annotation before truly adding a trailing comma.