MAINT: Uses copysign to extract direction of order.

Instead of using division of the amount by itself to extract
the direction, uses math's copysign.

Should be almost functionally equivalent,
but copysign won't have a possible floating point error leading
the direction to not be exactly 1.

zipline/finance/slippage.py

@@ -108,7 +108,7 @@ class VolumeShareSlippage(object):
             open_amount = order.amount - order.filled
 
             if(open_amount != 0):
-                direction = open_amount / math.fabs(open_amount)
+                direction = math.copysign(1, open_amount)
             else:
                 direction = 1