Make Set trait generic

[GruffElixir]

Fixes #927

Make Set preserve its element type parameter, matching List and the other container traits. This enables annotations such as values: Set[str] = Set() to expose Set[str] on the class and set[str] on instances.

Tests: python -m pytest tests/test_typing.py -q

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.21%. Comparing base (c4f1247) to head (b16490a).

Components	Coverage Δ
traitlets	85.63% <100.00%> (+<0.01%)	⬆️
tests	99.07% <ø> (ø)

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[GruffElixir]

The project tests, lint, Codecov, and downstream ipython/ipywidgets/nbconvert/notebook checks are passing. The two remaining failures appear external or transient: Read the Docs stopped during checkout with "fatal: reference is not a tree: b16490a", and the jupyter_server downstream job had one kernel-cull timing failure after 1,153 passes. Could a maintainer rerun those checks when convenient?