Use this buildpack with multipack if you have the following issue:
building '_cffi_backend' extension
creating build/temp.linux-x86_64-2.7
creating build/temp.linux-x86_64-2.7/c
gcc -pthread -fno-strict-aliasing -g -O2 -DNDEBUG -g -fwrapv -O3 -Wall -Wstrict-prototypes -fPIC -DUSE__THREAD -I/usr/include/ffi -I/usr/include/libffi -I/app/.heroku/python/include/python2.7 -c c/_cffi_backend.c -o build/temp.linux-x86_64-2.7/c/_cffi_backend.o
c/_cffi_backend.c:13:17: fatal error: ffi.h: No such file or directory
File .buildpacks:
https://github.com/Stibbons/heroku-buildpack-libffi.git
backend=https://github.com/heroku/heroku-buildpack-python.git
frontend=https://github.com/heroku/heroku-buildpack-nodejs