Make qtpy dependency optional

[Carifio24]

Downstream in glue-wwt we're trying to make qtpy be an optional dependency (glue-viz/glue-wwt#111) so that it doesn't need to be installed to use the WWT Jupyter viewer. To help make that possible, this PR makes qtpy an optional dependency of pywwt, moving it to the qt and qt6 install options.

The documentation already makes it sound like qtpy is an optional dependency, so I don't think it needs to be changed at all?

[Carifio24]

CI failures are the same as in #375; I can rebase once we finish sorting that out.

[pkgw]

Looks like you'll need to rebase to make ReadTheDocs happy.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 56.51%. Comparing base (5ec4686) to head (22117b7).
Report is 7 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #376   +/-   ##
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  Coverage   56.51%   56.51%           
=======================================
  Files          25       25           
  Lines        2872     2872           
=======================================
  Hits         1623     1623           
  Misses       1249     1249           

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