diff -up FreeCAD-0.18.4/cMake/FindPyCXX.cmake.unbundled-pycxx FreeCAD-0.18.4/cMake/FindPyCXX.cmake
--- FreeCAD-0.18.4/cMake/FindPyCXX.cmake.unbundled-pycxx	2019-10-22 12:53:35.000000000 -0400
+++ FreeCAD-0.18.4/cMake/FindPyCXX.cmake	2020-06-03 11:38:52.881038874 -0400
@@ -121,6 +121,7 @@ if(PYCXX_FOUND)
             ${PYCXX_SOURCE_DIR}/cxx_exceptions.cxx)
         add_definitions(-DPYCXX_6_2_COMPATIBILITY)
     endif()
+    add_definitions(-DPYCXX_PYTHON_2TO3)
 else(PYCXX_FOUND)
     MESSAGE(STATUS "PyCXX not found")
 endif(PYCXX_FOUND)