bpo-37305: add MIME type for Web App Manifest

[filips123]

Add .webmanifest -> application/manifest+json to list of recognized file types and content type headers.

Note that application/manifest+json is currently not registered with IANA (but it will be soon), so I can move it to common_types if needed.

Issue: https://bugs.python.org/issue37305

https://bugs.python.org/issue37305

[the-knights-who-say-ni]

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[filips123]

I signed the CLA today so it is currently not updated.

[filips123]

Ok, it is now confirmed to be signed.

[tirkarthi]

I think you have also reformatted the dictionary and the indentation change makes the diff larger than the one line change done.

[filips123]

@tirkarthi I indented all lines because .webmanifest key was too long.

[jaraco]

This change looks right to me.

[jaraco]

I notice that previously the mapping was ordered by value alphabetically. We should retain that form.

[bedevere-bot]

@jaraco: Please replace # with GH- in the commit message next time. Thanks!

[miss-islington]

Thanks @filips123 for the PR, and @jaraco for merging it 🌮🎉.. I'm working now to backport this PR to: 3.8.
🐍🍒⛏🤖

[bedevere-bot]

GH-15946 is a backport of this pull request to the 3.8 branch.