class apitools.base.protorpclite.messages.Message is unhashable in Python 3

The class `apitools.base.protorpclite.messages.Message` is an unhashable type in Python 3 and as a result, raises a TypeError when used as a key in a dict.

The reason for this is that in Python 3.x "a class that overrides `__eq__()` and does not define `__hash__()` will have its `__hash__()` implicitly set to None" (https://docs.python.org/3.6/reference/datamodel.html#object.__hash__). This makes it unhashable.

In Python 2, since `__eq__()` was defined, it simply inherited the `__hash__()` from object. In Python 3 it doesn't.