- restore Python 3.7 compatibility
- add behavior to handle returning additional `Page` subclasses to
continue scraping
- add default behavior when `Page.input` has a `url` attribute.
- add `PdfPage`
- add `page_to_items` helper
- add `Page.example_input` and `Page.example_source` for test command
- add `Page.logger` for logging
- allow use of `dataclasses` in addition to `attrs` as input objects
- improve output of HTML elements
- bugfix: not specifying a page processor on workflow is no longer an
error