Python: prep for abi3 work

Merged Ben Boeckel requested to merge ben.boeckel/vtk:abi3-prep into master

This MR updates Python code to use abi3-compatible APIs where possible. Note that there are a few WIP commits that contain comments that need decisions or other API updates to handle.


  • Test to see how badly this breaks Python2
  • Run test suite (compiles locally at least)

Cc: @banesullivan

Edited by Ben Boeckel

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