Needed parenthesis in withas

[rocky]

# Bug is not putting parens around (x, y)
def bug(self, nested, a, b):
    with self.assertRaises(ZeroDivisionError):
        with nested(a(), b()) as (x, y):
                1 // 0

when decompiled omits the parenthesis in as (x, y) and apparently this is needed.

rocky/python-uncompyle6@37b8e21

suggests what can be done here and in Issue #24