RSS verification fails for planet.opensuse.org

[KaratekHD]

Describe the bug
The RSS module does not parse the feed on https://planet.opensuse.org/global/rss20.xml. It fails with This link is not an RSS Feed link

To Reproduce
Steps to reproduce the behavior:

1. Open a private chat with the bot
2. Run /rss https://planet.opensuse.org/global/rss20.xml
3. See error

Expected behavior
The bot shows basic information about the feed file.

Screenshots

Version information (please complete the following information):

• Telegram Version: v8.4.2
• Nemesis Version: v2.2.0

Additional context
This problem occurs because the feed file in planet-o.o does not have the correct HTTP Header set (see openSUSE/planet-o-o#21 ). A potential fix for this would be to handle the feedparser.NonXMLContentType exception, as described in https://feedparser.readthedocs.io/en/latest/character-encoding.html#advanced-encoding.