python-cheetah: fix python host dependency

python-cheetah can't be used with python3. host-python-cheetah has the same
limitation.

Signed-off-by: Gwenhael Goavec-Merou <gwenhael.goavec-merou@trabucayre.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>