#5199 draw: vtk and vtk6 renderers invoke "python", which need not exist

[Ilia Gradina]

draw_renderer: vtk and draw_renderer: vtk6 run the generated script with
python, and the generated script asks for #!/usr/bin/env python. Since
PEP 394 that name is optional: Fedora only supplies /usr/bin/python through
the separate python-unversioned-command package, Debian dropped it in
bookworm, and Arch points it at python3. So on a machine where vtk7 and above
work, vtk and vtk6 die with

python: command not found

Nothing is left for the split to select. Python 2 has been end of life since
2020 and no current VTK builds bindings for it, so the python branch could
only ever have found python3 under an alias.

Patch attached (git format-patch, applies with git am); the same commit is
on

git://git.code.sf.net/u/ilgrad/maxima  branch vtk-python3

Seen on x86-64 Fedora 44 with VTK 9.5.2 and Maxima 5.50.0; the full test suite
with share_tests: true is unchanged by it.

[Ilia Gradina]

Patch attached,
file: 0001-draw-run-the-generated-vtk-script-with-python3.patch