AAA06: comment in Act block

Problematic code

def test() -> None:
    shopping = ['apples', 'bananas', 'cabbages']

    # Reverse shopping list operates in place
    shopping.reverse()  # act

    assert shopping == ['cabbages', 'bananas', 'apples']

Correct code

def test() -> None:
    """
    Reverse shopping list operates in place
    """
    shopping = ['apples', 'bananas', 'cabbages']

    shopping.reverse()  # act

    assert shopping == ['cabbages', 'bananas', 'apples']

Rationale

The Act block carries out a single action on an object. It is the focus of each test. Therefore any comments on this single action are really comments on the test itself and so should be moved to the test docstring.

By placing these important comments in the docstring we can:

Make it easier to keep the Act block simple.
Help to distinguish the Act block from the rest of the test.
Improve the documentation of tests because any important comments and notes are lifted to the top of the test.

Exceptions

Inline comments used to pass information to linters are OK:

Marking the Act block:
```
shopping.reverse()  # act
```

Marking lines in the action for linting reasons:

result = shopping.reverse()  # type: ignore