BUG: Py3 '-2j' is (NZERO - 2j) causes test failures

On python2.x evaluating '-2j' produces complex(0,-2), however, on
python3, it instead produces complex(NZERO, -2).  This ends up causing
a test for the complex arctan to fail.

This happens I suppose for the same reason it does in C.  The expression
-2j ends up being evaluated as (-2 + 0j) * (0 + 1j) == (-0 + -2j).