silx.gui.plot: Added Python typing #1285

Job	Run time
Python 3.7 on ubuntu-20.04 PyQt5 sdist	8m 58s
Python 3.7 on ubuntu-20.04 PyQt5 sdist	8m 58s
Python 3.10 on macos-latest PyQt5 wheel	8m 9s
Python 3.10 on macos-latest PyQt5 wheel	8m 9s
Python 3.8 on ubuntu-latest PySide6 sdist	8m 6s
Python 3.8 on ubuntu-latest PySide6 sdist	8m 6s
Python 3.9 on macos-latest PySide6 wheel	12m 44s
Python 3.9 on macos-latest PySide6 wheel	7m 10s
Python 3.7 on ubuntu-latest PyQt6 wheel	7m 27s
Python 3.7 on ubuntu-latest PyQt6 wheel	7m 27s
Python 3.10 on macos-latest PyQt6 wheel	12m 17s
Python 3.10 on macos-latest PyQt6 wheel	7m 53s
Python 3.9 on windows-latest No GUI wheel	6m 0s
Python 3.9 on windows-latest No GUI wheel	6m 0s
	1h 57m 24s

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