BF : workaround for broken complex256 on Python 3

Bizarre errors with byteswapped versions of complex256, apparently only
on Python 3.3, with numpy 1.7.0 - but - we'll see.

Errors were in `test_arraywriters`, claiming that two complex256 arrays
differing only in native, swapped endianness compared unequal. Debugging
showed that numpy was finding nans in the byteswapped array, and
subtraction results were odd, etc.