Set VTK 9.0 as the minimum supported version

[mwtoews]

Overview

As discussed in #3938 this PR removes pre-VTK 9 logic, setting VTK 9.0 as the minimum supported version.

Details

• Plenty of chops and simplifications where pre-VTK 9 logic was enabled
• Changes to pyvista/_vtk.py are difficult to see due the change of indent level, but are relatively minor
• Keep pyvista._vtk.VTK9 for now, as this is used externally by pytest-pyvista; remove pyvista._vtk.VTK91 which had limited use
• From pyvista/utilities/cells.py, remove generate_cell_offsets_loop() and generate_cell_offsets(), which were only used for pre-VTK9 UnstructuredGrid() with four arguments
• With the plotting renderer classess, remove the can_process_events property and other variables like VERY_FIRST_RENDER and update_timer_id
• Update docstrings and examples, where relevant
• Remove comments, notes or other remarks about previous versions
• Removed the needs_vtk9 pytest marker

[codecov]

Codecov Report

Merging #4018 (656dc39) into main (9b24a21) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #4018      +/-   ##
==========================================
+ Coverage   94.51%   94.52%   +0.01%     
==========================================
  Files          97       97              
  Lines       20609    20464     -145     
  Branches     3480     3430      -50     
==========================================
- Hits        19478    19344     -134     
+ Misses       1130     1119      -11     
  Partials        1        1              

[MatthewFlamm]

I started pyvista/pytest-pyvista#44 to enable this PR.

[banesullivan]

This is fantastic and I am eager to merge this when everyone is on board!

[MatthewFlamm]

Some comments to get started.

[MatthewFlamm]

This PR will be increasingly hard to maintain as other PRs are merged. I recommend merging soon if agreed upon. I made pyvista/pytest-pyvista#47 for a release then this PR can be finalized.

[akaszynski]

This is good to go. Thanks @mwtoews.

Let's merge this soon to avoid any potential merge conflicts.

[MatthewFlamm]

A forward looking thought: I'm not sure whether it would be now better to from vtkmodules import ... where it is used. _vtk now mostly just flattens vtkmodules.

[akaszynski]

A forward looking thought: I'm not sure whether it would be now better to from vtkmodules import ... where it is used. _vtk now mostly just flattens vtkmodules.

This is a great point. One issue we've had is that we load everything all upfront despite not using all libraries. If we instead "lazy load" everything, we can improve load time and memory footprint.