Warn on unrecognized Configurable kwargs with UserWarning

[DSeaStar]

Fixes #926.

HasTraits.__init__ already warns when leftover kwargs are passed through to object.__init__. The warning used DeprecationWarning at stacklevel=2, so for Configurable subclasses the reported frame is Configurable.__init__ and IPython hides it.

@minrk noted that this is always a bug in the calling code and that UserWarning is appropriate. This switches the category so the message is visible by default, including for Configurable / IPython.embed() typos.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.15%. Comparing base (1dae189) to head (3d606be).

Components	Coverage Δ
traitlets	85.50% <ø> (ø)
tests	99.07% <100.00%> (+<0.01%)	⬆️

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[DSeaStar]

The downstream / jupyter_server failure looks pre-existing and unrelated to this change: the same job fails on main (e.g. the latest Tests run on main, run 31784339759, has downstream / jupyter_server → failure, and main has been red on that job every day since Aug 9).

The failing test there is jupyter_server's own tests/test_utils.py::test_check_version (packaging.version.InvalidVersion: Invalid version: 1.0), caused by the newer packaging release rejecting float versions — nothing to do with the UserWarning for unrecognized kwargs added here. All other jobs on this PR are green.

[DSeaStar]

Update on the second failing check: downstream / downstream_check is not a real test job — it is the alls-green aggregation job that merely mirrors the state of the five downstream jobs (see downstream_check in .github/workflows/downstream.yml, needs: [notebook, ipywidgets, jupyter_server, nbconvert, ipython]). It is red here solely because downstream / jupyter_server is red, and that same job fails identically on main (run 31784339759: tests/test_utils.py::test_check_version — packaging.version.InvalidVersion: Invalid version: 1.0, the same signature as on this PR). No action needed on this PR for either check.