Implement item-centric WikidataBot.run

All wikidata scripts need to get the item for a page.
Convert them to using run() and treat() semantics.
Adds 'treat_missing_item' keyword, so newitem.py can
indicate that it wants to perform custom handling of
missing items for a page.

Also fix bug 66523 for all scripts, including category.py.
The bug was introduced by 431cb77.

And fix all pep257 errors except missing docstrings.

Bug: 66523
Change-Id: Iaaaf3fa583a3e299899197ac9c67530f1972a861