silx.gui.plot.ImageStack: fix `_urlIndexes` initialization. #1718

Job	Run time
Python 3.8 on ubuntu-20.04 PyQt5	10m 16s
Python 3.11 on ubuntu-latest PyQt6	9m 34s
Python 3.12 on ubuntu-latest PySide6	8m 58s
Python 3.10 on macos-13 PyQt5	13m 53s
Python 3.12 on macos-13 PyQt6	16m 55s
Python 3.9 on macos-13 PySide6	12m 15s
Python 3.9 on windows-latest PyQt5	10m 41s
Python 3.12 on windows-latest PyQt6	11m 12s
Python 3.10 on windows-latest PySide6	8m 53s
	1h 42m 37s

Provide feedback