I am investigating a migration to Python 3, and to facilitate this we are...

I am investigating a migration to Python 3, and to facilitate this we are using the -3 flag as decribed here:
https://docs.python.org/3/howto/pyporting.html#prevent-compatibility-regressions . When using this flag I encountered
some issues inside of mock itself.

Python 3 now requires you to implement __hash__ if you implement __eq__. See https://docs.python.org/3.6/reference/datamodel.html#object.%5F%5Fhash%5F%5F .

```python
{mock.ANY}  # Fine in Python 2, Throws in Python 3
```

This PR explicitly sets the `__hash__` method on these objects as `None` to ensure the behavior is consistent in Python 3 as well as Python 2.