Add blog.python.org to Planet Python feed

[treyhunner]

I see the pyfound.blogspot.com feed is still in the config for https://www.planetpython.org. That feed should probably be changed to the blog.python.org version.

[hugovk]

https://pyfound.blogspot.com/ is the PSF blog and isn't yet migrated away from Blogger.

https://pythoninsider.blogspot.com/ is the Python Insider blog, which is in this repo. Previously, https://blog.python.org/ used to show the same as pythoninsider.blogspot.com.

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But we should update this line to point to the new feed: https://blog.python.org/rss.xml

https://github.com/python/planet/blob/3c782ed5261a728a5d4974e0e719d47a28cebaee/config/config.ini#L1158

I can review/merge things at https://github.com/python/planet but not my own PRs, so if someone else wants to create the PR, I can merge it :)

[JacobCoffee]

We should hold off on any changes to planet

[hugovk]

Issue about ditching feedburner: python/pythondotorg#2787