Rework attributes

[MaxDall]

This should only be merged after #148 and with the proper settings. Currently this commits into #148

This PR addresses issues raised in #138.

It does so by adding a parameter supported to the attribute decorator.

• If set to True (default), the decorated attribute is validated against the attribute_guidelines (name, annotation),
• If set to False the attribute is excluded from those tests.

This also does a major refactor on the article class. We have to so article supports unsupported attributes.

• Every parsed attribute is now accessible through Article, but only those supported will be actual fields of Article. See the following example

class TestParser(BaseParser):
    @attribute
    def body(self) -> ArticleBody:
        ...

    @attribute
    def authors(self) -> List[str]:
        ...

    @attribute
    def publishing_date(self) -> Optional[datetime.datetime]:
        ...

    ...

    @attribute(supported=False)
    def key_notes(self):
        ...

You can now access all attributes with dot notation (article.body, article.key_notes) but only those in the guidelines will have auto-completion, and providing no annotation for key_notes here won't raise any error duuring testing.

• some rework in the Article structure and inheritance.

[dobbersc]

I tried my best to review this PR. But be aware that I do not totally understand everything that has been changed.

[dobbersc]

Ah there is one parameter left to rename