py2: kpi: Fix tobytes on np.void

On py2 np.void.data is buffer which does not have .tobytes() .

kpi.py

@@ -352,8 +352,8 @@ def __eq__(a, b):
         return False
     # cast to dtype without aliases to avoid
     # "dtypes with overlapping or out-of-order fields are not representable as buffers"
-    return a.view(a._dtype0).data.tobytes() == \
-           b.view(b._dtype0).data.tobytes()
+    return bytes(a.view(a._dtype0).data) == \
+           bytes(b.view(b._dtype0).data)
 
 @func(Measurement)
 def __ne__(a, b):