`Element.attrs` iteration yields names but no values

```pycon
>>> element = markupever.parse("<html id=test lang=en>").root().first_child
>>> type(element.attrs)
<class 'markupever.dom.AttrsList'>
>>> element.attrs
[(QualName(local="id"), "test"), (QualName(local="lang"), "en")]
>>> list(element.attrs)
[QualName(local="id"), QualName(local="lang")]
```