BUG: Add Python3 CMake module workarounds to ITKModuleExternal
BUG: Add Python3 CMake module workarounds to ITKModuleExternal

Ported from ITK/CMakeLists.txt

This prevents missing required Python related variables especially when
wrapping.

malaterre/ITK,BRAINSia/ITK,BRAINSia/ITK,thewtex/ITK,hjmjohnson/ITK,malaterre/ITK,LucasGandel/ITK,richardbeare/ITK,vfonov/ITK,hjmjohnson/ITK,LucasGandel/ITK,Kitware/ITK,thewtex/ITK,vfonov/ITK,BRAINSia/ITK,vfonov/ITK,richardbeare/ITK,BRAINSia/ITK,Kitware/ITK,thewtex/ITK,malaterre/ITK,Kitware/ITK,hjmjohnson/ITK,vfonov/ITK,thewtex/ITK,LucasGandel/ITK,thewtex/ITK,richardbeare/ITK,richardbeare/ITK,thewtex/ITK,InsightSoftwareConsortium/ITK,InsightSoftwareConsortium/ITK,Kitware/ITK,LucasGandel/ITK,hjmjohnson/ITK,InsightSoftwareConsortium/ITK,hjmjohnson/ITK,Kitware/ITK,vfonov/ITK,vfonov/ITK,richardbeare/ITK,vfonov/ITK,vfonov/ITK,Kitware/ITK,LucasGandel/ITK,richardbeare/ITK,InsightSoftwareConsortium/ITK,LucasGandel/ITK,BRAINSia/ITK,hjmjohnson/ITK,malaterre/ITK,malaterre/ITK,malaterre/ITK,richardbeare/ITK,malaterre/ITK,BRAINSia/ITK,LucasGandel/ITK,malaterre/ITK,BRAINSia/ITK,vfonov/ITK,Kitware/ITK,LucasGandel/ITK,hjmjohnson/ITK,malaterre/ITK,InsightSoftwareConsortium/ITK,InsightSoftwareConsortium/ITK,InsightSoftwareConsortium/ITK,thewtex/ITK