bpo-43194: Add JFXX as jpeg marker to imghdr.py

[alexdraconian]

Add support for JFIF extension APP0 marker ("JFXX" Header) in imghdr.

https://bugs.python.org/issue43194

[the-knights-who-say-ni]

Hello, and thanks for your contribution!

I'm a bot set up to make sure that the project can legally accept this contribution by verifying everyone involved has signed the PSF contributor agreement (CLA).

Recognized GitHub username

We couldn't find a bugs.python.org (b.p.o) account corresponding to the following GitHub usernames:

@alexdraconian

This might be simply due to a missing "GitHub Name" entry in one's b.p.o account settings. This is necessary for legal reasons before we can look at this contribution. Please follow the steps outlined in the CPython devguide to rectify this issue.

You can check yourself to see if the CLA has been received.

Thanks again for the contribution, we look forward to reviewing it!

[tirkarthi]

Please open an issue at https://bugs.python.org since this is a code change that will require discussion.

[github-actions]

This PR is stale because it has been open for 30 days with no activity.

[github-actions]

Closing this stale PR because the CLA is still not signed.