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[maint] Defer importing scipy.sparse (#2744)

Followup to: https://github.com/vispy/vispy/pull/2726
part of the efforts to improve napari startup time:
https://github.com/napari/napari/issues/8689

Importing a visual also imports scipy.sparse via
vispy.visuals ->
vispy.visuals.graphs -> vispy.visuals.graphs.layouts.random ->
vispy.visuals.graphs.util -> scipy.sparse

In this PR I remove the duplicated code in `force_directed.py` and defer
the import of scipy.sparse.

main, second run:
`python -X importtime -c 'from vispy.visuals import VolumeVisual' 2>&1 |
grep scipy`
`import time:       250 |      73988 |             scipy.sparse`

this branch same command returns nothing.

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Co-authored-by: Lorenzo Gaifas <brisvag@gmail.com>

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