support Qt6

[tlambert03]

can't merge until qtpy releases v2... still some debugging of combo boxes left

[codecov]

Codecov Report

Merging #324 (911fe8b) into main (12f78d2) will increase coverage by 0.00%.
The diff coverage is 82.35%.

@@           Coverage Diff           @@
##             main     #324   +/-   ##
=======================================
  Coverage   90.36%   90.37%           
=======================================
  Files          29       29           
  Lines        3363     3365    +2     
=======================================
+ Hits         3039     3041    +2     
  Misses        324      324           

Impacted Files	Coverage Δ
magicgui/backends/_qtpy/widgets.py	86.37% <80.00%> (+0.03%)	⬆️
magicgui/backends/_qtpy/application.py	100.00% <100.00%> (ø)

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