This is because buildozer search cython, but the actual command is cython3. To solve this you have to create a cython command that passes all arguments to cython3.

Write in the terminal

cd /bin/ && sudo gedit cython
in the editor that will pop-up write

cython3 $@
this will pass all the arguments from cython to cython3. Save the file and write in the terminal

sudo chmod 755 cython
to make it executable. To test it simply write

cython
It should have the same output as

cython3