This release includes 1cdcd85 from #1411 to fully support usage of the latest VTK wheels, thus (finally) enabling support of Python 3.9 for pyvista.
Note that to use the pre-release wheels, you must specify the version of the package with:
pip install vtk==9.0.20210612.dev0
Note that this is unnecessary on Python 3.9 as pip will use unstable or development wheels if they are the only ones available.