vtkRenderingMatplotlib: support interpreter-less builds

Ben Boeckelrequested to merge
ben.boeckel/vtk:matplotlib-no-interp-req into master

In a wheel, there is no point in running our own interpreter because it will already be provided. Instead, add checks to ensure that Python is initialized (no matter how) before calling any Python APIs.

Cc: @banesullivan

Edited by Ben Boeckel

Merge request reports