fix: deprecated use of traitlets

[maartenbreddels]

No description provided.

[codecov-commenter]

Codecov Report

Merging #368 (a5d9511) into master (150ff1c) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #368   +/-   ##
=======================================
  Coverage   70.86%   70.86%           
=======================================
  Files          23       23           
  Lines         834      834           
=======================================
  Hits          591      591           
  Misses        243      243           

Impacted Files	Coverage Δ
pythreejs/traits.py	90.83% <100.00%> (ø)

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[vidartf]

The way I read this, it will also mean a drop of support for traitlets 4?

[maartenbreddels]

Hmm, i didn't think about that actually.