Bump vtk from 8.1.2 to 9.0.1

Bumps [vtk](https://vtk.org) from 8.1.2 to 9.0.1.

---
updated-dependencies:
- dependency-name: vtk
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

requirements.txt

@@ -15,7 +15,7 @@ pytest-cov
 devito
 jupyter-book
 matplotlib
-vtk==8.1.2
+vtk==9.0.1
 mayavi
 ipyevents
 ipywidgets