ENH: Fixup headers, add single file exporter (Sourcery refactored)

[sourcery-ai]

Pull Request #31 refactored by Sourcery.

If you're happy with these changes, merge this Pull Request using the Squash and merge strategy.

NOTE: As code is pushed to the original Pull Request, Sourcery will
re-run and update (force-push) this Pull Request with new refactorings as
necessary. If Sourcery finds no refactorings at any point, this Pull Request
will be closed automatically.

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To manually merge these changes, make sure you're on the fixupHeaders branch, then run:

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Help us improve this pull request!

[sourcery-ai]

Function export_single refactored with the following changes:

• Remove unnecessary calls to enumerate when the index is not used (remove-unused-enumerate)