Get package authors from author_email if author is not provided

[provinzkraut]

For packages that make use of the "new" authors field in pyproject.toml, their authors are currently not displayed, as https://pypi.org/pypi/<package>/json returns authors under author_email in this case.

This is a simple fix by using author_email as a fallback, and extracting the names from there.

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I think I successfully tested this locally :)