com_factor():  when a unary minus is attached to a float or imaginary zero,
don't optimize the UNARY_MINUS opcode away:  the const dict can't
distinguish between +0.0 and -0.0, so ended up treating both like the
first one added to it.  Optimizing UNARY_PLUS away isn't a problem.

(BTW, I already uploaded the 2.2a3 Windows installer, and this isn't
important enough to delay the release.)