Improve import speed

[michalk8]

Description

See if we can lazily-load napari/pyqt5, think these should be the main culprits (need to verify).

[ivirshup]

For verification, I think profimp was useful for scanpy. We also added tests to make sure slow imports aren't added back in.

Previous discussion in scanpy might be useful here: https://github.com/theislab/scanpy/issues/756

[giovp]

we'll deprecate napari soon so this should be no problem

[giovp]

deprecate napari should go in 1.3 already