Exporting to BytesIO in same fashion as to file

[HyTurtle]

Popping the fmt out of the kwargs to pass it on to the save method; which is needed to save to a BytesIO object.

Maintainer merge checklist

• [ ] Title is descriptive/clear for inclusion in release notes.
• [ ] Applied a Component: xxx label.
• [ ] Applied the api-deprecation or api-break label.
• [ ] Applied the release-highlight label to be highlighted in release notes.
• [ ] Added to the milestone version it was merged into.
• [ ] Unittests are included in PR.
• [ ] Properly documented, including versionadded, versionchanged as needed.

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