5.6.1 builds with python 3.4.x import error

@ben.boeckel @demarle

Tested with superbuild tag v5.6.1-python3 using v5.6.1 paraview tag.

Python 3.4.9 was used in this build.

pvpython
>>> import vtk
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/projects/paraview/stam/07-26-19/paraview-sdk/5.6.1-py3-osmesa_intel-18.0.2_openmpi-2.1.2_python-system3/lib/python3.4/site-packages/vtk.py", line 31, in <module>
    all_m = importlib.util.module_from_spec(all_spec)
AttributeError: 'module' object has no attribute 'module_from_spec'

A superbuild patch is in for this at my branch: ethan.stam/paraview-superbuild@f966590d

This patch needs to go in the v5.6.1-python3 tag if that's possible.