Python 3 preparation: use // and __truediv__ as needed. Tool-assisted patch. Those divisions that I thought must be integer floor divisions and rely on Python 2 integer floor division semantics are changed to use // (if any are actually meant to be floating-point divisions, that would break things). One __div__ method is changed to __truediv__ (with __div__ = __truediv__ for Python 2 compatibility).

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