Small integer powers of real±0j are invalid

For example,

>>> z1 = complex(1,+0.0)
>>> z1**2
(1+0j)
>>> z1*z1
(1+0j)
>>> z2 = complex(1,-0.0)
>>> z2**2
(1+0j)
>>> z2*z2
(1-0j)

"Big enough" or non-integer powers are unaffected:

>>> z2**2000
(1-0j)
>>> z2**2.1
(1-0j)

It seems, this is related to

Line 164 in c179c0e

r = c_1;

Instead, imaginary component of r should copy the sign from x.imag.

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