test_iop fix: assign res to in-place operator function #130

[simonetgordon]

To resolve issue #130

[asmeurer]

This looks good. This probably didn't come up for NumPy because it actually does a mutation, but that is not a requirement of the spec. In Python, x op= y is equivalent to x = x.__iop__(y) (see https://docs.python.org/3/reference/datamodel.html?highlight=__iadd__#object.__iadd__).