Import traitlets as top-level module

[romanngg]

This change helps fix issues like https://stackoverflow.com/questions/75666380/attributeerror-module-ipython-utils-traitlets-has-no-attribute-unicode

I noticed that the rationale

# Importing via IPython raises a spurious warning, but avoids a version
# mismatch internally.

dates back 4 years ago, so wondering if it's not an issue no longer?
Thank you.

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