Scikit-SurgeryVTK 2.0.0 Release: compatible with vt v9.2.6 and PySide6 v6.5.0

Summary -- Release highlights

• Updated latest versions of packages requirements (vtk>=9.2.6, PySide6>=6.5.0, opencv-contrib-python-headless>=4.7.0.72).
• Debugged 163 unit tests locally and remotely in a systematic-like manner
• Passed local and remote tests for python versions 3.8, 3.9, 3.10, 3.11. This is a summary for python 3.8:
  • local ubuntu22.04x64 (154 passed, 9 skipped, 13 warnings in 25.55s)
  • CI ubuntu-latest, 3.8 (156 passed, 7 skipped, 13 warnings in 81.63s)
  • CI windows-latest, 3.8 (161 passed, 2 skipped, 13 warnings in 68.24s)
  • CI macos-latest, 3.8 (161 passed, 2 skipped, 13 warnings in 189.63s)
• Publish pypi final release 2.0.0 https://pypi.org/project/scikit-surgeryvtk/#history
• Update classifiers in setup.py https://pypi.org/classifiers/

Docs Fixes

• Fixed building docs errors with python3.8 by setting up admin in readthedocs.org, adding .readthedocs.yaml, requirements-docs.txt and updating docs/conf.py, and
• Warnings from building docs were reduced from 201 to 26

Other Bug Fixes

• Sorted out qt plugin errors by exporting QT settings in ci.yml
• updates ci.yml with Qt for X11 Requirements> https://doc.qt.io/qt-6/linux-requirements.html