Add Qt6 support to pygui

[jamesjhu]

Support discovery for PyQt6 and PySide6 in pygui().

[codecov-commenter]

Codecov Report

Merging #1002 (9839383) into master (b27bd5a) will decrease coverage by 0.26%. The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master    #1002      +/-   ##
==========================================
- Coverage   68.36%   68.09%   -0.27%     
==========================================
  Files          20       20              
  Lines        2023     2031       +8     
==========================================
  Hits         1383     1383              
- Misses        640      648       +8     

Flag	Coverage Δ
unittests	68.09% <0.00%> (-0.27%)	:arrow_down:

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/gui.jl	0.00% <0.00%> (ø)
src/PyCall.jl	70.69% <0.00%> (ø)

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[stevengj]

LGTM.