Fix Python 3 support in unit package.

wrappers/python/simtk/unit/quantity.py

@@ -77,6 +77,7 @@ import math
 import copy
 from .standard_dimensions import *
 from .unit import Unit, is_unit, dimensionless
+import sys
 
 class Quantity(object):
     """Physical quantity, such as 1.3 meters per second.
@@ -818,6 +819,8 @@ def _is_string(x):
      except StopIteration:
          return False
 
+if sys.version_info >= (3,):
+    del Quantity.__nonzero__
 
 # run module directly for testing
 if __name__=='__main__':