Make it possible to import glue-wwt without Qt installed

[astrofrog]

Because glue-wwt can be used in a Jupyter context, we should:

• Make qtpy an optional dependency
• Add a -qt factor in tox or even pyqt and pyside ones like in glue-qt and then have a mix of builds in CI, some with PyQt, some with PySide, and some without either (and maybe with jupyter stuff I guess)
• Make sure the test suite skips tests it can't run if Qt isn't available

[Carifio24]

We already have qtpy as an optional dependency, and the newest release of pywwt does as well, so the first item should be all set.

[Carifio24]

@astrofrog While working on this I realized that while we've removed any qtpy dependency from pywwt, we're still pulling in qtpy as it's a dependency of both echo (on which this package depends) and of pvextractor (pulled in from glue-core).

I would think that for echo we should be able to make qtpy an optional dependency, as I imagine it's not needed unless one is using the Qt widget <--> callback linking functions, but I'm pretty unfamiliar with the functionality of pvextractor so I'm not sure how key it is there.

[astrofrog]

echo and glue-core now no longer pull in qtpy, so this should now be possible?

[Carifio24]

I believe so! I'll check and verify that, and if so I can start addressing the latter two items in the list.

[astrofrog]

Thanks!