There are a few common multi CPU libraries that are used for numerical computations, including inside of NumPy. There are a few environment flags that you can set before running the script to limit the number of CPUS that they use.
Try setting all of the following:
export MKL_NUM_THREADS=1
export NUMEXPR_NUM_THREADS=1
export OMP_NUM_THREADS=1
Sometimes it’s a bit tricky to see where exactly multithreading is introduced.
Other answers show environment flags for other libraries. They may also work.