Fix compatibilty with Qt6

[astrofrog]

Overview

@Carifio24 I tested out your Qt6 fix suggested in #343 and it works fine when using PyWWT inside glue, so from my perspective, this is a sufficient fix.

Checklist

• Changed code has some test coverage (or justify why not)
• Changes in functionality documented (or justify why not)

[astrofrog]

I don't understand the CI setup well enough to be able to add a CI job with PyQt6

[codecov]

Codecov Report

Merging #346 (40118e8) into master (4ae8627) will decrease coverage by 0.01%.
The diff coverage is 83.33%.

❗ Current head 40118e8 differs from pull request most recent head 9878543. Consider uploading reports for the commit 9878543 to get more accurate results

@@            Coverage Diff             @@
##           master     #346      +/-   ##
==========================================
- Coverage   56.17%   56.16%   -0.01%     
==========================================
  Files          25       25              
  Lines        2836     2838       +2     
==========================================
+ Hits         1593     1594       +1     
- Misses       1243     1244       +1     

Impacted Files	Coverage Δ
pywwt/qt.py	69.59% <83.33%> (-0.24%)	⬇️

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[pkgw]

I'll see if I can add a CI job although I might not get to it super promptly. If this is sufficiently urgent I'd be OK with merging now and mopping that up later.

[astrofrog]

@pkgw - it would be nice to have this in soon as currently I am not able to use glue-wwt correctly on native Python installations on ARM macs (only PyQt6 is supported), but I'll leave it up to you whether to merge now/later.

[pkgw]

OK, this has been released in 0.18.1.

[astrofrog]

Thank you!