old_contents: from distutils.core import setup
  from distutils.extension import Extension
  from Cython.Distutils import build_ext
  from Cython.Build import cythonize
  setup(name = 'math_3d', ext_modules = cythonize("math_3d.pyx"),)
  
new_contents: from distutils.core import setup
  from distutils.extension import Extension
  from Cython.Distutils import build_ext
  from Cython.Build import cythonize
  setup(name = 'math_3d', ext_modules = cythonize("math_3d.pyx"),)
  setup(name = 'billboard', ext_modules = cythonize("billboard.pyx"),)
  
subject: add billboard.pyx to cython build