Should check for cython3

[ZXTube]

I'm on a kali linux and it seems that whenever you pip install cython on a linux machine it gets called cython3

Before Buildozer gives the error
# Cython (cython) not found, please install it.
It should check if there's a cython3

The current work around for this though is running this: sudo ln -s $(which cython3) /usr/local/bin/cython

[Julian-O]

Buildozer doesn't support Cython 3 yet, because python-for-android doesn't yet.