ColormapDialog: upgrade `_DataInPlotMode` to `DisplayMode` #1301

Job	Run time
Python 3.7 on ubuntu-20.04 PyQt5 sdist	4m 20s
Python 3.10 on macos-latest PyQt5 wheel	4m 28s
Python 3.8 on ubuntu-latest PySide6 sdist	4m 10s
Python 3.9 on macos-latest PySide6 wheel	3m 45s
Python 3.7 on ubuntu-latest PyQt6 wheel	4m 15s
Python 3.10 on macos-latest PyQt6 wheel	4m 23s
Python 3.9 on windows-latest No GUI wheel	4m 19s
	29m 40s

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