Upgrade feedparser to version 6.0.8

[ben-rowan]

Relates to #38

Upgrades feedparser to version 6.0.8. This version supports Python:

• 3.6
• 3.7
• 3.8
• 3.9

And allows podfox to install correctly using setup.py

ref: https://pypi.org/project/feedparser/6.0.8/

[dalepotter]

@brtmr Can this be merged?

Without it, it seems podfox cannot be installed using setuptools>=58 (such as in the GitHub Actions runners, whic are running setuptools v59.6.0 by default). Additionally Podfox will not work on >=py39 without this change merged,