Parallel/Core/Testing: fix `TestSocketCommunicator` execution

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

Fixes: #19542

.gitlab/ci/ctest_exclusions.cmake

@@ -4,9 +4,7 @@ set(test_exclusions
   # https://gitlab.kitware.com/vtk/vtk/-/issues/19427
   "^VTK::RenderingOpenGL2Cxx-TestGlyph3DMapperPickability$"
   # https://gitlab.kitware.com/vtk/vtk/-/issues/19544
-  "^vtkRenderingParallel-TestClientServerRendering$"
-  # https://gitlab.kitware.com/vtk/vtk/-/issues/19542
-  "^vtkParallelCore-TestSocketCommunicator$")
+  "^vtkRenderingParallel-TestClientServerRendering$")
 
 if (NOT "$ENV{CMAKE_CONFIGURATION}" MATCHES "windows")
   list(APPEND test_exclusions

Parallel/Core/Testing/Cxx/CMakeLists.txt

@@ -11,7 +11,7 @@ if (Python3_EXECUTABLE)
     vtkParallelCore-TestSocketCommunicator TestSocketCommunicator.cxx)
 
   add_test(NAME vtkParallelCore-TestSocketCommunicator
-           COMMAND ${PYTHON_EXECUTABLE}
+           COMMAND ${Python3_EXECUTABLE}
                    ${VTK_SOURCE_DIR}/CMake/vtkTestDriver.py
                    # indicate that the processes are not run sequentially,
                    --process