The feedparser packages has 20 years of history and is very good at parsing
RSS and Atom, so use that instead of ad-hoc regex and XML parsing.
The medium_rss and shaarli_rss parsers weren't touched because they are
probably unnecessary. (The special parse for pinboard is just needing because
of how tags work.)
Doesn't include tests because I haven't figured out how to run them in the
docker development setup.
Fixes#1171
`item.find(p)` returns either an `ElementTree.Element` or `None`. The
[lambda on line 24][lambda] coerces the return value to a bool, which is
`False` if the `<link>` element has no children (see
[`ElementTree.py` line 207][etbooldef]), so the lambda returns `None`.
Further, returning a `Link` with `url=None` violates
[an assertion in `index/schema.py`][assertion], which crashes
the `archivebox add` command.
[lambda]: 3d54b1321b/archivebox/parsers/pinboard_rss.py (L24)
[etbooldef]: 3d8993a744/Lib/xml/etree/ElementTree.py (L207)
[assertion]: 3d54b1321b/archivebox/index/schema.py (L165)
