Undable to build 9.0.0.rc1 under python 3.6

We are working on a test build of 9.0.0.rc1 on conda-forge: https://github.com/conda-forge/vtk-feedstock/pull/105 The build is a work in progress but one error I have been seeing with master and remains with 9.0.0.rc1 is that python 3.6 is not being detected correctly:

-- Could NOT find Python3 (missing: Development) (found suitable version "3.7", minimum required is "3.2")
CMake Error at CMake/vtkModule.cmake:4112 (message):
  Could not find the Python3 external dependency.
Call Stack (most recent call first):
  Utilities/Python/CMakeLists.txt:44 (vtk_module_find_package)

My builds with osmesa and python 2.7, 3.7 and 3.8 get past this point without any trouble.

I suspect the problem could be with CMake rather than VTK but I am not experienced enough with CMake to reproduce the problem there.

Any help would be very much appreciated, particularly before 9.0.0 is released so we make sure it can be build successfully for conda-forge.