BUG: Set Python3_EXECUTABLE on Windows
BUG: Set Python3_EXECUTABLE on Windows

The FindPython3 CMake module fails to set the Python3_EXECUTABLE
variable to the discovered value on Windows -- only _Python3_EXECUTABLE
is set.

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