fix VtkViewerNew for VTK 9.4.0

[ManuelHu]

this should fix:

FAILED tests/fluka/test_Fluka.py::test_fluka_vis - TypeError: unhashable type: 'PolyData'
FAILED tests/visualisation/test_Convert.py::test_vtkPolyDataToNumpy - TypeError: unhashable type: 'PolyData'
FAILED tests/visualisation/test_Visualisation.py::test_VtkViewerNewAppend - TypeError: unhashable type: 'PolyData'
FAILED tests/visualisation/test_Visualisation.py::test_VtkViewerColouredNewAppend - TypeError: unhashable type: 'PolyData'
FAILED tests/visualisation/test_Visualisation.py::test_VtkViewerColouredMaterialNewAppend - TypeError: unhashable type: 'PolyData'

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.

Project coverage is 74.11%. Comparing base (cf097c1) to head (e0ebd94).

Files with missing lines	Patch %	Lines
src/pyg4ometry/visualisation/VtkViewerNew.py	0.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #218      +/-   ##
==========================================
- Coverage   74.12%   74.11%   -0.01%     
==========================================
  Files         157      157              
  Lines       22735    22735              
==========================================
- Hits        16853    16851       -2     
- Misses       5882     5884       +2     

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[ManuelHu]

also limits sphinx-autoapi version to < 3.4.0 (see readthedocs/sphinx-autoapi#504)