Fix handling of single font name in theme (#7967)
* Fix handling of single font name in theme
Previous check failed because `str` is an instance of Iterable so was never correctly wrapped in `[...]`. Instead, providing a single font such as "Arial" would result in a font list of "A, r, i, a, l" in the bundled CSS.
* Handle single fonts.Font instance correctly
* add changeset
* changes
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Co-authored-by: gradio-pr-bot <gradio-pr-bot@users.noreply.github.com>
Co-authored-by: Ali Abid <aliabid94@gmail.com>
