I want to render math equation in my application. The minimal code is:

```
import vtkmodules.all as vtk
text = vtk.vtkTextActor()
text.SetInput('Cell1 | Cell2 | $\\sum_{i=0}^\\infty x_i$\n12 | $\\sum_{i=0}^\\infty x_i$ | 2345')
text.SetDisplayPosition(100, 100)
property = text.GetTextProperty()
property.SetFontSize(20)
render = vtk.vtkRenderer()
render.AddActor(text)
renWin = vtk.vtkRenderWindow()
renWin.AddRenderer(render)
iren = vtk.vtkRenderWindowInteractor()
iren.SetRenderWindow(renWin)
iren.Initialize()
iren.Start()
```

It can correctly render the math equation as following:

However, it also report a warning:

```
MatplotlibDeprecationWarning:
The MathtextBackendBitmap class was deprecated in Matplotlib 3.4 and will be removed two minor releases later. Use mathtext.math_to_image instead.
iren.Start()
```

Why this warning is reported?

The environment is:

```
win 10
python 3.9.12
vtk 9.1.0
matplotlib 3.5.1
```