Fix: allow feedparser 6.0.2 to solve issues in Python 3.9

[apmarshall]

Python 3.9 changes the way base64.decodestring works, which results in AttributeErrors in gitsome. Expanding the version range for feedparser to include 6.0.2 solves these issues. Addresses #186

[Jimimaku]

Python 3.9 ändert die Art und Weiseworks, was zu gitsome führt. Die Erweiterung des Versionsbereichs für Feedparser löst diese Probleme. Adressen#186base64.decodestring``AttributeErrors``6.0.2

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 53.21%. Comparing base (d7c57ab) to head (868401c).
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Additional details and impacted files

@@             Coverage Diff             @@
##           master     #187       +/-   ##
===========================================
- Coverage   95.09%   53.21%   -41.89%     
===========================================
  Files          34       86       +52     
  Lines        2119     6585     +4466     
===========================================
+ Hits         2015     3504     +1489     
- Misses        104     3081     +2977     

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