Remove visible mocks in documented types

[MatthewJA]

Looking at the autodocs, we see things like:

class acton.database.ASCIIReader(
    path: str, feature_cols: typing.List[str],
    label_col: str,
    encode_labels: bool = True,
    label_encoder: <MagicMock name='mock.LabelEncoder' id='140295020343192'> = None)

It would be great if we could somehow hide that mock. Perhaps overriding __repr__ for mocked documentation types would be a possibility?