Python complains about a syntax error during installation.
```
/usr/lib/python3/dist-packages/vtkmodules/web/testing.py:687: SyntaxWarning: invalid escape sequence '\.'
fnamePattern = re.compile("([^\.\/\\\]+)\.py")
/usr/lib/python3/dist-packages/vtkmodules/web/testing.py:687: SyntaxWarning: "\." is an invalid escape sequence. Such sequences will not work in the future. Did you mean "\\."? A raw string is also an option.
fnamePattern = re.compile("([^\.\/\\\]+)\.py")
```
`"([^\.\/\\\]+)\.py"` isn't a valid string and the resulting regular expression only works by accident. It should be replaced with `r"([^./\\]+)\.py"` to get the intended behavior.