Fix minor typos in What's New in Python 3.8.

[jerobado]

Added periods at the end of the sentences.

[the-knights-who-say-ni]

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[tirkarthi]

I am not sure if this is always enforced. cc: @rhettinger

[miss-islington]

Thanks @jerobado for the PR, and @serhiy-storchaka for merging it 🌮🎉.. I'm working now to backport this PR to: 3.8.
🐍🍒⛏🤖

[bedevere-bot]

GH-16835 is a backport of this pull request to the 3.8 branch.

[serhiy-storchaka]

Thank you @jerobado!