Implement array trait, drop traittypes dependency

[chrishavlin]

traittypes is unmaintaned (last code modification 4 years ago) and is the source of a pytest warning we've been getting for a while:

../../../../../opt/hostedtoolcache/Python/3.9.20/x64/lib/python3.9/site-packages/traitlets/__init__.py:28
  /opt/hostedtoolcache/Python/3.9.20/x64/lib/python3.9/site-packages/traitlets/__init__.py:28: DeprecationWarning: 
              Sentinel is not a public part of the traitlets API.
              It was published by mistake, and may be removed in the future.

This is a minimal replacement for traittypes.Array that I think covers what we need. The main thing I did not add is any explicit dtype handling -- I could, but yt_idv isn't using any at the moment so I didn't add it (but don't think it'd be hard)... I also could refactor YTPositionTrait to inherit from the new ArrayTrait, but thought it'd be better to make that as a subsequent change.

[matthewturk]

Can we also try submitting a PR to traittypes?

[chrishavlin]

just so i don't forget and accidentally merge this, gonna switch to draft til we decide what to do re: upstream PRs. This PR is a pretty low priority -- it's unlikely that traitlets will actually deprecate the Sentinel class anytime soon.