Fix RubricCollector.clear_doc()

[mattheww]

I've seen occasional errors like the following on incremental builds:

Extension error (ferrocene_spec.items_with_rubric):
Handler <bound method RubricCollector.clear_doc of <ferrocene_spec.items_with_rubric.RubricCollector object at 0x7f4a44a47e50>> for event 'env-purge-doc' threw an exception (exception: pop index out of range)

The following code in exts/ferrocene_spec/items_with_rubric.py

    # This makes a copy of the list (with `list(items)`) to be able to
    # remove items from it without affecting the iteration.
    for i, item in enumerate(list(items)):
        if item.document == docname:
            items.pop(i)

won't work as intended if it finds more than one item to remove, because the first pop will change the positions of the following elements of the list.

[tshepang]

bors merge

[tshepang]

what's the advantage of doing it this way, instead of...

items = [item for item in items if item.document != docname]

[tshepang]

(answering self) it modifies the list instad of creating a new one

[tshepang]

I suppose the generator is coerced to a list as well (but I don't know the right Python terminology), while the choice of generator is for being gentle to memory, where the items are created only when needed

[bors-ferrocene]

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