More Typing Adoption

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• Gets us closer to the goal of strict typing with the adoption of the ANN check in ruff, which lets us more easily toggle where to enforce typing.

• Moves the tests directories into a top level tests folder.

• Adds a regression test for __doc__ no longer contains the traitlets help string for reference type traitlets (Dict, List, ...) #869

• Fixes handling of Type trait and adds tests, partially addresses More typing edge cases #871

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tests/test_traitlets.py is a pure file move but it is showing up as a new file, maybe because it is so big.

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Oh duh, the big diff is because we had to leave a few items in traitlets/tests/test_traitlets.py that are used downstream.