* Backport traverse_obj and closely related function from yt-dlp (code by pukkandan)
* Backport LazyList, variadic(), try_call (code by pukkandan)
* Recast using yt-dlp's newer traverse_obj() implementation and tests (code by grub4k)
* Add tests for Unicode case folding support matching Py3.5+ (requires f102e3d)
* Improve/add tests for variadic, try_call, join_nonempty
Co-authored-by: dirkf <fieldhouse@gmx.net>