For this task you can clone the master branch of class in a directory class/ on the same level as this one, either from the public repository:
cd ..
git clone https://github.com/lesgourg/class_public.git class/
or the private one:
cd ..
git clone https://github.com/lesgourg/class.git class/
then:
cd class
make clean
PYTHON=python3 make -j
cd ../Euclid_KP_nu