Use feedparser for RSS parsing in generic_rss and pinboard_rss parsers

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

pyproject.toml

@@ -15,6 +15,7 @@ dependencies = [
     "dateparser>=1.0.0",
     "django-extensions>=3.0.3",
     "django>=3.1.3,<3.2",
+    "feedparser>=6.0.11",
     "ipython>5.0.0",
     "mypy-extensions>=0.4.3",
     "python-crontab>=2.5.1",