pyvista: running PyVista in sphinx examples causes segfault on a clean docs build

When building the docs, a segfault occurs if its a clean build:

cd ./docs/
rm -r _build
rm -r examples
make html

note that a segfault occured. Then if you build again, no segfualt occurs

make html

This is an issue for vtki and other projects running vtki code in their docs/examples.

Note that in our .travis.yml, we build the docs twice to avoid this issue.

I have a feeling it has to do with some sort of object cleanup not happening - noticed this after implementing the sphinx_gallery scraper