Build failure due to expander.py

When building on Mac, I get the failure:

[1/440] Checking validity of Tuple.h
FAILED: vtkm/Tuple.h.checked 
cd /Users/4nt/vtk-m/build/vtkm && /Applications/CMake.app/Contents/bin/cmake -DPYTHON_EXECUTABLE=/Applications/Xcode.app/Contents/Developer/Library/Frameworks/Python3.framework/Versions/3.7/bin/python3.7 -DPYEXPANDER_COMMAND=/usr/local/bin/expander.py -DSOURCE_FILE=/Users/4nt/vtk-m/vtkm/Tuple.h -DGENERATED_FILE=/Users/4nt/vtk-m/build/vtkm/Tuple.h -P /Users/4nt/vtk-m/CMake/testing/VTKmCheckPyexpander.cmake
Traceback (most recent call last):
  File "/usr/local/bin/expander.py", line 13, in <module>
    import pyexpander.lib as pyexpander
ModuleNotFoundError: No module named 'pyexpander'
CMake Error at /Users/4nt/vtk-m/CMake/testing/VTKmCheckPyexpander.cmake:48 (message):
  Running pyexpander failed (1).

However,

$ pip3 install pyexpander
Requirement already satisfied: pyexpander in /usr/local/lib/python3.7/site-packages (1.10.2)

Note that the Python version is correct. Moreover,

➜  vtkm git:(refactor_integrators) ✗ which expander.py
/usr/local/bin/expander.py
➜  vtkm git:(refactor_integrators) ✗ expander.py Tuple.h.in > Tuple.h
➜  vtkm git:(refactor_integrators) ✗ echo $?
0

It appears the problem is caused by the line -DPYTHON_EXECUTABLE=/Applications/Xcode.app/Contents/Developer/Library/Frameworks/Python3.framework/Versions/3.7/bin/python3.7; for example I can replicate the problem via

 /Applications/Xcode.app/Contents/Developer/Library/Frameworks/Python3.framework/Versions/3.7/bin/python3.7 `which expander.py`
Traceback (most recent call last):
  File "/usr/local/bin/expander.py", line 13, in <module>
    import pyexpander.lib as pyexpander
ModuleNotFoundError: No module named 'pyexpander'

I attempted for fix via adding -DPYTHON_EXECUTABLE=/usr/local/bin/python3, but no joy:

CMake Warning:
  Manually-specified variables were not used by the project:

    PYTHON_EXECUTABLE