Change generation of child mocks

- https://docs.python.org/3/library/unittest.mock.html

We previously used an override of the `__new__` method to prevent our
custom mock types from instantiating their children with their own
type instead of a general mock type like `MagicMock` or `Mock`.

As it turns out, the Python documentation suggests another method of
doing this that does not involve overriding `__new__`. This commit
implements this new method to make sure we're using the idiomatic way
of handling this.

The suggested method is overriding the `_get_child_mock` method in
the subclass. To make our code DRY, I've created a mixin that should
come BEFORE the mock type we're subclassing in the MRO.

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In addition, I have also added this new mixin to our `AsyncMock`
class to make sure that its `__call__` method returns a proper mock
object after it has been awaited. This makes sure that subsequent
attribute access on the returned object is mocked as expected.

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