Python doctests: test for None

Other option would be a direct check for None:

def six_or_none(val):
    """
    >>> six_or_none(6)
    6
    >>> six_or_none(4)
    """
    if val == 6:
        return 6
    return None

The Python interpreter ignores None return values, so doctests do the same.

Test for is None instead:

>>> six_or_none(4) is None
True

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