Use qtpy Signal in DiscreteColorWidget for PySide6 compatibility

[fjorka]

Problem

import napari_spatialdata fails under PySide6:

ImportError: cannot import name 'pyqtSignal' from 'pyqtgraph.Qt.QtCore'

_scatterwidgets.py takes pyqtSignal (a PyQt-only name) from pyqtgraph's shim for DiscreteColorWidget.paletteChanged. Since __init__.py imports _view → _scatterwidgets, the whole package is unimportable under PySide6, not just the scatter widget. These are the only PyQt-only names in the codebase, so this is the complete fix.

Fix

Use qtpy's Signal, already imported in this file and already used by ScatterListWidget.attrChanged.

Verification

• PyQt5: import succeeds before and after (no regression).
• PySide6: fails before, succeeds after.

Context

I introduced this in #270; it went unnoticed because the all extra installs napari[pyqt5].
Same breakage as #397, this is a minimal internal fix.