Check vtkmodules and vtk in sys.modules

[xavArtley]

Fix CI errors relative to VTKVolume

[codecov]

Codecov Report

Merging #2071 (b7af42e) into master (c816ad0) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #2071   +/-   ##
=======================================
  Coverage   84.43%   84.43%           
=======================================
  Files         178      178           
  Lines       20636    20636           
=======================================
  Hits        17423    17423           
  Misses       3213     3213           

Impacted Files	Coverage Δ
panel/pane/vtk/vtk.py	96.39% <100.00%> (ø)

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[philippjfr]

Thanks, really appreciate the quick turnaround.