Quadratic Behavior in xml.etree.ElementPath Index Predicates

`Element.findall()` and fully-consumed `Element.iterfind()` exhibit `O(n^2)` time complexity when using XPath index predicates (e.g. `[1]`, `[last()]`, `[last()-N]`) on XML documents with many same-tag siblings. `Element.find()` is only affected when the first match is near the end  of the sibling list, such as with `[last()]` or `[last()-N]`;  `.//item[1]` short-circuits after the first match.