Rendering/Parallel/Testing: fix `TestClientServerRendering` execution

Use the new `FindPython3` module's variable for the Python executable.

Fixes: #19544

.gitlab/ci/ctest_exclusions.cmake

@@ -2,9 +2,7 @@ set(test_exclusions
   # Flaky when run with threads enabled. See #19471.
   "^VTK::FiltersCellGridCxx-TestCellGridEvaluator$"
   # https://gitlab.kitware.com/vtk/vtk/-/issues/19427
-  "^VTK::RenderingOpenGL2Cxx-TestGlyph3DMapperPickability$"
-  # https://gitlab.kitware.com/vtk/vtk/-/issues/19544
-  "^vtkRenderingParallel-TestClientServerRendering$")
+  "^VTK::RenderingOpenGL2Cxx-TestGlyph3DMapperPickability$")
 
 if (NOT "$ENV{CMAKE_CONFIGURATION}" MATCHES "windows")
   list(APPEND test_exclusions

Rendering/Parallel/Testing/Cxx/CMakeLists.txt

@@ -23,7 +23,7 @@ if (Python3_EXECUTABLE)
 
   ExternalData_add_test(${_vtk_build_TEST_DATA_TARGET}
     NAME vtkRenderingParallel-TestClientServerRendering
-    COMMAND ${PYTHON_EXECUTABLE}
+    COMMAND ${Python3_EXECUTABLE}
       ${VTK_SOURCE_DIR}/CMake/vtkTestDriver.py
       --process
         $<TARGET_FILE:vtkRenderingParallel-TestClientServerRendering>